The leading GOP presidential candidates unleashed an all-out ad war going into the final days before the Iowa caucuses and New Hampshire primary, each trying to fight multiple candidates on multiple fronts in an effort to hold on to gains in the polls.

And the ads have triggered harsh closing words from candidates on the stump.

Like a boxer hearing the bell for the final round, Mike Huckabee came out swinging going into the weekend, responding to Mitt Romney's new ad in Iowa criticizing his record on government spending, immigration and crime and even sticking his neck out for rival John McCain.

"I think it is because he has spent millions and millions of dollars ... and I am sure there is a feeling of frustration. You are not supposed to be behind when you spend that kind of money," Huckabee said of Romney in Indianola, Iowa, Saturday.

"Mitt doesn't have anything to stand on except to stand against," he said earlier in the day at Boyt Harness Co., a hunting accessory business in Osceola.

But a new poll shows the race in flux before the Jan. 3 caucuses. While other recent surveys have put Huckabee slightly ahead of Romney in Iowa, an American Research Group poll out of the state showed Romney with 32 percent and Huckabee with 23 percent. The poll was taken of 600 likely GOP caucus-goers from Dec. 26-28, and had a margin of error of 4 percent.

The final days of the race are playing out like a dead heat. Huckabee, who kept a plain-spoken, folksy approach to politics as he rocketed skyward in the polls, dropped the nice-guy tone over the weekend.

"When people get that far behind, having spent that much money, they get desperate," Huckabee said Friday. "Desperate is one thing but dishonest is something else, and when you get desperate and dishonest it's not a very pretty sight."

The former Arkansas governor also responded to Romney’s criticism of McCain, calling the former prisoner of war an "honest-to-God American hero" worthy of respect. Romney's ad in New Hampshire slammed McCain for his opposition to Bush tax cuts and for his positions on illegal immigration. Romney released another ad Saturday in New Hampshire criticizing him for being lax on immigration.

"It's one thing to attack me, but it just kind of made me get a little bit concerned that if that's all we're going to do in the Republican Party is attack each other then maybe all we're doing is making it a whole lot easier for the other side to win the White House next year," Huckabee said.

He did not stop there. Taking aim at Romney’s abortion stance, Huckabee told his audience: "I know that Mr. Romney has said that he's pro-life, but that's a new position for him. But just before he left office he signed a health care bill that provides an elective abortion for a $50 co-pay ... We must stand clear and firm that human life has value and meaning and is precious to us."

In defense of the Romney ad calling him soft on crime, Huckabee pointed to the 16 people he sent to death row, saying Romney never had to do that and touting his tenure as governor as a time when Arkansas crime rates fell. He said Saturday that he denied 90 percent of 8,700 clemency requests as governor.

The Romney ad says Huckabee is "soft on government spending" and portrays him as weak on immigration and crime. But Huckabee launched an ad of his own where he says, "If you love negative campaigning, you have got to be loving the last few days of this election. But if you love our country, you have got to be thinking enough is enough."

About 10 candidates from both parties are running ads in Iowa. Candidates widely pulled back for a truce in Iowa during Christmas, but the tension is back with so few days to make their cases before the caucuses. Romney is fighting a battle on two fronts, as Huckabee challenges him in Iowa, and McCain has come from behind to pose a threat in New Hampshire, which holds its primary Jan. 8.

Romney ostensibly played it safe Saturday with a series of quick coffee-shop stops where he delivered a final pitch that was long on Iowa platitudes but short on substance. But the former Massachusetts governor's brief remarks -- filled with storytelling, chitchat and bullet points of his agenda -- belied the cutthroat nature of the GOP race and contrasted sharply with his hard-hitting television ads.

The campaign attempted to respond to the backlash from his rivals.

Romney spokesman Eric Fehrnstrom said in a statement Saturday that the former Massachusetts governor "has been talking about the issues and avoiding personal attacks. The fact is Mike Huckabee supports in-state tuition for illegal aliens, he increased spending and taxes as governor of Arkansas and he gave out 1,033 pardons and commutations to criminals."

And in Iowa Friday evening, Romney said: "So our ad is honest and it describes what he did, what his positions were as governor," Romney said. "They happen to be different than mine. So if he has an argument, you know, saying something isn't honest is also a form of attack that really isn't necessary. " 

Later Saturday, Romney took what may have been a veiled swipe at his opponents , saying "I'm going all over the country making sure that I get this nomination. I'm not just a one-hit wonder."

Romney also found himself responding to a counterattack from McCain, who launched an ad in New Hampshire that sought to cast the former Massachusetts governor as untruthful. The ad quoted a recent Concord Monitor editorial suggesting Romney was "phony."

McCain earned the endorsement Saturday from the Monitor, which praised him for "his honesty and his refusal to pander."

On a bus in Iowa, Romney said his ad "contrasts my position" with McCain's, while McCain's ad was downright mean.

"I make no attacks on his character, no attacks of a personal nature whatsoever. I've just seen the text of his ad and it is obviously of a very different nature. It's an attack ad. It attacks me personally. It's nasty. It's mean-spirited," he said. "Frankly, it tells you more about Sen. McCain than it does about me that he would run an ad like that."

McCain said Saturday that the "back and forth" will come to an end.

"We responded, but we're not going to continue to go back and forth. But when somebody spends a whole bunch of money portraying my position on major issues ... in an inaccurate fashion, obviously we need to respond to it."

After former Pakistani Prime Minister Benazir Bhutto was assassinated Thursday, McCain has tried to engage voters in the faraway turmoil, arguing the United States needs a president with his experience to deal with international crises.

McCain opened several town hall meetings with comments on Pakistan Saturday. He told crowds that the situation in Pakistan should matter to voters because it has implications for national security at home.

And he released a statement after Usama bin Laden issued an audiotape vowing to expand Al Qaeda's holy war to Israel.

"Osama bin Laden's latest propaganda message and the assassination of Benazir Bhutto are stark reminders that we are engaged in a worldwide conflict with Islamist extremism, and we cannot relent or waver," McCain said.

Meanwhile, former New York City Mayor Rudy Giuliani stayed out of the fray Saturday. Giuliani is still leading slightly in most national GOP polls, and has concentrated on voter-rich states like Florida and the Feb. 5 primary states. But he's trailing badly in Iowa, and campaigned in the Hawkeye State Saturday.

In an interview with The Associated Press, Giuliani dismissed suggestions that his campaign is stalled.

"I think we are very relevant," Giuliani said. "We are challenging in more states than anybody else."

FOX News' Serafin Gomez, Malini Bawa, Carl Cameron and Shushannah Walshe and The Associated Press contributed to this report.