The Republican presidential nominating contest took on a new shape Thursday with an additional candidate in the race and new life injected into a suffering campaign following a praiseworthy showing at a primary debate in New Hampshire.

Former Tennessee Sen. Fred Thompson officially joined the 2008 presidential campaign with an announcement made just after midnight on the "Tonight Show With Jay Leno" followed by an Internet Webcast on his own campaign Web site.

"We're where we need to be right now, and that's one of the things I wanted to talk to you about. I'm running for president of the United States," Thompson told Leno.

Meanwhile, Arizona Sen. John McCain won wide praise from voters participating in an instant response meter that gauged reaction to candidates' remarks. The nearly unanimous reaction came after a retooling of McCain's campaign following several months of poor fundraising, a staff shake-up and falling poll numbers.

Thompson, who several times this year delayed entering the campaign and has been plagued by campaign staffing changes even before a campaign was in place, said he started thinking about entering the race in March. He rejected assertions that his late start would hurt his chances.

"I don't think people are going to say, you know, 'That guy would make a very good president, but he just didn't get in soon enough.' Communications being what they are nowadays, if you can't get your message out in a few months, you're probably not ever going to get it out," Thompson said.

The former "Law & Order" star made his official entry announcement just after the other eight candidates seeking the Republican nod wrapped up a debate marked by both banter and bickering.

Anticipating Thompson's entry, the other candidates welcomed him into the contest but wondered about his timing.

"I think he's done a really good job of playing my part on 'Law and Order.' I personally prefer the real thing but I think Fred will add something to the race," former New York Mayor Rudy Giuliani, who was once a U.S. attorney, said at the debate on FOX News held at the University of New Hampshire's Whittemore Center.

"I was scheduled to be on Jay Leno tonight, but I gave up my slot for somebody else because I'd rather be in New Hampshire with these fine people," said former Arkansas Gov. Mike Huckabee.

"Why the hurry? Why not take some more time off?" added former Massachusetts Gov. Mitt Romney.

Thompson's campaign immediately claimed that the candidate was dominating the Republican presidential primary process, releasing news headlines that noted his absence from the debate. Thompson himself told Leno that he doesn't think much of the current debate structures.

"The segments now, you know, you've got 10 guys if everybody shows up, you know, with 30, 40 second sound bites. It's not designed to enlighten the American people," he said.

"And I'll tell you something else. For those who talk about that New Hampshire situation, I'm certainly not disrespecting them, but it's a lot more difficult to get on the 'Tonight Show' than it is to get into a presidential debate," Thompson added.

Surge in Iraq, Surge for McCain

While Thompson's absence was noted several times throughout the evening, candidates found plenty of room to talk about several issues — and spent a predominant amount of time arguing over the degree of success that has occurred as a result of the troop surge in Iraq. The tone offered a far cry from Democratic debates or even earlier Republican debates in which words like "redeployment" and "withdrawal" dominated the discussion.

• Click here to read the full transcript of the debate.

In Wednesday night's debate, McCain, a Vietnam war hero, was repeatedly praised by the other candidates, who agreed with many of his remarks on the prosecution of the war but didn't go as far as McCain in emphasizing the success of the surge in Iraq.

"It is working. No, not 'apparently'; it's working," McCain said, taking issue with Romney over his cautious optimism.

McCain said the surge has been successful in part because of a good strategy being executed by Gen. David Petraeus, the head of Multinational Forces in Iraq who is set to testify to Congress next week about the successes of the Iraqi government to meet 18 military and political benchmarks. Critics of the congressionally-mandated report say the benchmarks are worded to ensure failure and scheduled on an impossible timeframe. The general's remarks are sure to be used in the debate on Capitol Hill over changes in U.S. force structure in Iraq.

"The Maliki government is not doing the things we want it to do, the police are not functioning the way we want them to do, but we are succeeding. And the great debate is not whether it's apparently working or not, the great debate is going to take place on the floor of the United States Senate the middle of this month. And it's going to be whether we set a date for withdrawal, which will be a date for surrender, or whether we will let this surge continue and succeed," McCain said.

McCain added that he wants the troops to return home — when it's the right time for the United States to leave.

"I want our troops home with honor. Otherwise, we will face catastrophe and genocide in the region," he said.

On another topic, McCain also scored well among his peers and the audience when he said he pledged to "veto every pork barrel bill that comes across my desk. And I will make the authors of those pork barrel projects famous, and that's what I've been doing for a lot of years."

Several of the candidates praised McCain throughout the debate, with Huckabee saying that "if there's anybody on this stage that understands the word honor, I've got to say Sen. McCain understands that word.

"He has given his country a sacrifice the rest of us don't even comprehend, and on this issue, when he says we can't leave until we've left with honor, I 100 percent agree with him" on Iraq, he said.

"I have tremendous respect for Senator McCain. I think I've said, more than once, if I wasn't running I'd probably be supporting him for president," said Giuliani, who took similar positions as McCain on Iraq and a no-new taxes pledge that both called unnecessary.

Even Thompson on the Tonight Show called McCain a "good friend."

"John McCain and I sat side by side on the Senate floor. He's a good friend and will be after this is over with unless, of course, he beats me," he said.

But not all the candidates shared McCain's insistence on the rightness of the war, and Texas Rep. Ron Paul led the criticism.

"I'm the anti-war candidate representing the Republican traditional position," he said before getting into one of the night's liveliest sparring matches — a back-and-forth with Huckabee over whether it's time for the United States to leave Iraq.

"Going into Iraq and Afghanistan and threatening Iran is the worst thing we can do for our national security. I am less safe, the American people are less safe for this. It's the policy that is wrong," Paul said.

"The people who say there will be a bloodbath are the ones who said it will be a cakewalk or it will be a slam dunk, and that it will be paid for by oil. Why believe them?" he asked.

In response, Huckabee said that it was agreed before the war started that if the U.S. breaks Iraq, it must buy it.

"Congressman, whether or not we should have gone to Iraq is a discussion the historians can have, but we're there. We bought it because we broke it. We've got a responsibility to the honor of this country and to the honor of every man and woman who has served in Iraq and ever served in our military to not leave them with anything less than the honor that they deserve," Huckabee said.

Paul then responded: "The American people didn't go in. A few people advising this administration, a small number of people called the neoconservatives hijacked our foreign policy. They're responsible, not the American people."

Huckabee retorted that the United States is one nation. "We can't be divided. We have to be one nation, under God. That means if we make a mistake, we make it as a single country: the United States of America, not the divided states of America," he said.

"No, when we make a mistake — when we make a mistake, it is the obligation of the people, through their representatives, to correct the mistake, not to continue the mistake," Paul replied.

"And that's what we do on the floor of the Senate," Huckabee said.

California Rep. Duncan Hunter ended the discussion by saying progress by U.S. forces in Iraq should determine whether the United States should remain there.

"We've got 129 battalions in the Iraqi army that we're training up. We're training them up, we are getting them into the fight. When those Iraqi battalions are battled-hardened and they start to rotate into the positions on the battlefield displacing American forces, the American forces can then rotate out, come back to the U.S., or go to other places in Central Command. That's the right way to win. It's called victory. That's how we leave Iraq," he said.

National Security, Domestic Responsibilities

But not all the debate on stage was on Iraq. Colorado Rep. Tom Tancredo called Iraq a battlefield in the War on Terror but the departure of U.S. forces does not mean the end to threats to national security. He said if he were president, he would take whatever action necessary short of torture to defend the U.S.

"I would do — certainly, waterboard — I don't believe that that is, quote, 'torture.' I would do what is necessary to protect this country. That is the ultimate responsibility of the president of the United States. All of the other things that we do, all of the other things — all of the other powers vested in him pale in comparison to his responsibility to keep the people of this country safe. And that is ultimate. And, yes, I would go to great lengths to keep this country safe."

Tancredo, whose presidential campaign platform has been based almost solely on stopping illegal immigration, also lashed out at the other candidates for political correctness, saying that is what "will get us all killed." He offered another retort to those who change their views based on when the "wind is blowing in one direction."

"I'd like to see more than rhetoric ... (enforcing immigration law) has got nothing to do with disliking people entering this country, it has to do with the rule of law. Does anyone understand that?" Tancredo said.

On the topic of immigration, McCain defended his position as a co-sponsor of an immigration reform bill this year that failed in the Senate largely because of popular opposition to the guest worker program, called "amnesty" by opponents, that was outlined in the plan.

"Amnesty, according to the dictionary, is 'forgiveness.' The proposal that we had would require fines, would require back in the line, would require deportation for some. It would require others to go back to the country of their origin. It would require an enormous amount before anyone, as long as 13 years, could even be eligible for citizenship in this country," McCain said.

"Why we failed is because the American people have lost trust and confidence in us — our failure in Katrina, our failures in Iraq, our failures to control runaway spending. ... There's 12 million people who are in this country illegally, which is de facto amnesty, and we need a temporary worker program. I commit to securing the borders first. We can secure those borders. As president, I would have the border state governors certify that those borders were indeed secure," he said.

Romney, who ran an ad criticizing Giuliani for being mayor of a "sanctuary city" — one that does not prosecute illegal immigrants in New York City, also defended his enforcement of immigration law as governor of Massachusetts, saying that he could not force mayors to follow state law.

"With regards to sanctuary cities, the governors aren't responsible for mayors who are not following the law. And, actually, in my case, as soon as I learned about a program in the department of ICE (Immigration and Customs Enforcement) that we could have our state police authorized to enforce the law, I did just that so that in sanctuary cities in our state — and nonsanctuary cities — the law would be enforced," Romney said.

Giuliani countered that his policy on illegal immigrants enabled him to clean up crime in the city even if it meant leaving illegals in place.

"The problem that I had was I had 400,000 illegal immigrants, roughly, in New York City. And I had a city that was the crime capital of America. I had to do something intelligent with them," he said. "So what I did was, I said — and I think this a sensible policy: If you are an illegal immigrant in New York City and a crime is committed against you, I want you to report that. Because lo and behold, the next time a crime is committed, it could be against a citizen or a legal immigrant."

Giuliani also used a similar defense to explain his support for gun control laws, though said what worked for New York City may not be the best answer for places like Virginia Tech, where a shooter killed 33 students and faculty last spring.

"I think states have a right to decide that, states have a right to decide their gun laws. The Second Amendment grants you the right to bear arms. We have a federal system," he said.

Paul cited the Sept. 11, 2001, terror attacks as an example of the failure of the U.S. government to protect Americans by enacting too strict gun laws.

"Here is one example when the federal government was involved and they messed it up, and if we put the responsibility on the right people, respected the Second Amendment, I sincerely believe there would have been a lot less chance of 9/11 ever happening," he said.

On social issues, Huckabee said he didn't care to compare the positions of Giuliani and Romney on abortion, and the debate about abortion doesn't revolve around whether it should be a federal or state issue.

"The reason this country has been extraordinarily interested in what's going on to those miners out in Utah is because even though we don't know them, they represent us in the sense that they are human beings, and we don't know their fate. We need to show the same kind of respect for life whether a child is in the womb, or whether in a coal mine, or in a long-term care facility," he said.

Hitting on another socially sensitive topic, Kansas Sen. Sam Brownback received a mix of applause and boos when he said he supports a constitutional amendment banning gay marriage.

"When you do these vast, social experiments — and that's what this is, when you redefine marriage — it's a vast, social experiment. They're not done in isolation. They impact the rest of the culture around you. When you take the sacredness out of marriage, you will drive the marriage rates down," he said.

Brownback, who has tried to corner the socially conservative base of the GOP, said he thought Idaho Sen. Larry Craig, who pleaded guilty last month to a misdemeanor plea of disorderly conduct for allegedly soliciting a cop in a men's bathroom at a Minnesota airport in June, ought to stick to his commitment to resign. That position was echoed by Hunter.

"I think he ought to stick with the commitment that he made. And, you know, that's one thing about our party. When our guys have problems like this, they leave. They leave the Senate or they leave the House. When the Democrats have problems like this, they often make them chairmen of their respective committees," said Hunter of California.

Asked about his own family values, Giuliani, whose personal life includes two divorces and sometimes frothy relationships with his children, said he's running as an executive who knows how to get definable results in situations that people think are impossible to fix.

"Obviously, any issues in my private life do not affect my public performance," he said, adding that he is "not running as the perfect candidate for president of the United States," he's "running as a human being."

Among the would-be panelists in the debate were Republican voters at Young's Restaurant, who posed their questions remotely. One sheriff's deputy whose son is due back from Iraq told Romney that he was out of line for comparing his sons' campaigning for Romney to be president to the service performed by U.S. troops.

"There is no comparison, of course," Romney replied. "There's no question but that the honor that we have for men and women who serve in our Armed Forces is a place of honor we will never forget and nothing compares to it."