Michael Hayden told senators weighing his possible future Thursday that the National Security Agency's warrantless wiretap program is legal and always has been.

Hayden led the National Security from 1999 to 2005 and helped create the terror surveillance program whose legality is now under question. He was on Capitol Hill to answer a full day of questions in open and closed-door sessions that are supposed to help senators decide on his confirmation as CIA director.

Senate Intelligence Committee Chairman Pat Roberts said he wants a panel vote on the nominee early next week.

Sen. Russ Feingold offered the most forceful opposition of the terror surveillance program, saying he has concluded, following a classified briefing Wednesday, that the program is illegal.

"I came away from that briefing yesterday more convinced than ever first that the program is illegal and second that the president misled the country in 2004 before the revelations about this program became public when he said that wiretapping of Americans in this country requires a warrant," Feingold, D-Wis., said.

Several senators asked why officials didn't try to change the law so eavesdropping on suspected terrorists in domestic-international calls would be clearly legal. Hayden said members of the Intelligence Committee discussed that but decided it was a bad idea.

"There was a powerful and general consensus that an attempt to change the legislation would lead to revelations about the nature of the program and thereby hurt its operational effectiveness," Hayden said.

In defending the program, Hayden said the privacy of American citizens is a "concern constantly."

"When I had to make this personal decision in October 2001 ... the math was pretty straightforward. I could not not do this," he told members of the Senate Intelligence Committee.

"We always balance privacy and security and we do it within the law," he added. "I can certainly understand why someone would be concerned. ... The privacy of American citizens is a concern constantly, it's a concern in this program, and it's a concern in everything we've done."

The Air Force general said that since launching the program a month after the Sept. 11, 2001, terror attacks, policy-makers in the U.S. government who are most knowledgeable about Al Qaeda have been making the decisions about which calls are subject for scrutiny.

"There is a probable cause standard. Every target is documented" with a lengthy check list, he said.

Click here to read Gen. Hayden's opening statement (pdf).

The Bush administration has tried to keep a lid on NSA activities though two programs have leaked to the press. Under Hayden's leadership, the NSA listened without warrants to telephone calls when one party was overseas and suspected of terrorism. Though the administration has neither confirmed nor denied it, news reports also claim a second program took millions of phone records from telephone companies and tracked the origin and destination of numbers of interest.

Administration officials have repeatedly insisted that everything being done is legal — and that no domestic phone calls are being monitored without a warrant. In recent polls, Americans have suggested that they are less worried about this than are some politicians.

Officials also have said that it is has kept appropriate lawmakers informed about NSA's activities since the terror surveillance program began. They have argued that they didn't want to bring more people in the loop for fear of classified information leaking from Congress.

In an attempt to pave the way for smoother hearings for Hayden, John Negroponte, the director of national intelligence, authorized complete briefings on Wednesday to all members of the House and Senate Intelligence Committees on whatever it is that the NSA is doing regarding the monitoring of phone calls and phone call records.

But Feingold said he sees no reason why administration officials did not discuss the warrantless wiretap program with the entire Senate Intelligence Committee years ago.

"This long-overdue briefing hastily arranged on the evening of this nomination in my view does not provide enough assurance that the administration's general contempt for congressional oversight has diminished," he said. "There was absolutely no reason that the administration could not have told the full committee about the program four and a half years ago as is required by law."

Sen. Carl Levin, the committee's acting ranking Democrat while Sen. Jay Rockefeller recovers from back surgery, said leaks about the intelligence community's activities "are producing piecemeal disclosures."

Levin, of Michigan, said the Bush administration's response to leaks suggests that they are not being upfront about what activities the NSA is doing and whether those actions violate the privacy rights of Americans.

"It's not hard to see how Americans could feel that their privacy has been intruded upon if the government has, as USA Today reports, a database of phone numbers calling and being called by tens of millions of Americans who are not suspected of any wrongdoing," Levin said.

"And it's certainly not hard to see the potential for abuse and the need for an effective check in law on the government's use of that information," he added.

"I now have a difficult time with your credibility," said Sen. Ron Wyden, D-Ore.

Chairman Roberts said the NSA surveillance programs have proven themselves essential, although most of the public reporting has been wrong and members of the committee, prohibited by the classified status of the information, are unable to correct the record.

"We are literally unable to say anything ... the result I have found is that ignorance is no impediment for some critics," Roberts said.

During the hearing, Hayden restrained himself from answering several questions posed by lawmakers. He refused to talk publicly about news stories on broad NSA surveillance activities.

"I'm not at liberty to talk about that in open session," he said.

A career intelligence official, Hayden reached the penultimate of his 37-year government career when he took over at the NSA in 1999. In 2005, Hayden left the NSA to become deputy director of national intelligence.

The four-star general said if he were confirmed as director of the nation's pre-eminent human intelligence agency, he would make sure the CIA focuses on the job of spying and not politics.

The intelligence community in general "has too much become the football of American political discourse," Hayden said. "CIA needs to get out of the news as source or subject and focus on protecting the American people by acquiring secrets" from U.S. enemies.

"The public debates regarding your nomination have not been dominated by your record as a manager ... but rather the debate has focused almost entirely on the president's authorization" to do surveillance, Roberts noted.

Though his confirmation is said assured, Hayden faced tough questions from some committee members. Sen. Dianne Feinstein, D-Calif., asked Hayden's position on whether it's acceptable to strap prisoners to a plank and dunk them in water until they nearly drown, an interrogation technique called "waterboarding." Hayden wouldn't answer publicly and declined to comment about how long terror suspects should be held without a trial.

During a break, Feinstein complained that he had answered none of her questions, though Hayden told her he would respond when the committee headed into closed session in the late afternoon.

While the intelligence agencies are accused of violating Americans' rights in the conduct of their duties, their failure to obtain critical information in the past is blamed for terror attacks on the United States and its interests abroad.

Levin said the next director of the CIA "must right this ship and restore the CIA to its critically important position."

"One major question for me is whether General Hayden will restore analytical independence and objectivity at the CIA and speak truth to power," he said.

While NSA chief, Hayden was head of signals intelligence. In his opening statement, Hayden said if he were confirmed as the new director of the CIA, he would use his position to "enhance the standards of trade craft in human intelligence collection across the country."

Hayden said as head of CIA, he will instill a "tolerance for ambiguity and dissent manifested with a clear tolerance of judgments," a reference to accusations the administration suppressed conflicting information about Saddam Hussein's weapons capabilities before the war in Iraq.

Then-CIA chief George Tenet reportedly told President Bush that knowledge of Hussein's program was a "slam dunk," but those assumptions have since been debunked.

While "nobody bats 1,000 in the intelligence world," Roberts said the CIA pre-Iraq efforts demonstrated "a terribly flawed trade craft."

Hayden said that part of the reason for failure is the CIA was trying to do too much. He said they should not have been focusing on "over-achieving" or should not have gone "for moon shots."

Asked about U.S. intelligence on Iran, Hayden called that country "a difficult problem" and suggested U.S. intelligence-gathering on Iran's weapons program was more complex and detailed than that done on Iraq.

Hayden said questions raised included, "How are decisions made in that country? Who are making those decisions? What are their real objectives?"

Hayden told lawmakers that the CIA remains the central and most important intelligence agency in the U.S. arsenal. While he wants to work on improving that agency's technology and research development, the role of the agents has to be the most important goal.

"No other agency has the connective tissue to the other parts of the intelligence community that the CIA has ... though the CIA no longer manages the entire intelligence community, the director continues to lead the community in many key respects."

Hayden said as CIA director he would try to improve information sharing with foreign partners and improve the sense of collegiality among U.S. agencies.

"We welcome additional players on the field but they must work together as a team," he said.

Under questioning from Roberts, Hayden also indicated he believed analysts might need to take on more risk to collect information, but it comes at the risk of their sources and themselves.

"I think the agency itself would admit it is one of the more conservative elements of the community," Hayden said.

Military Leadership at the CIA

Bush chose Hayden as CIA director-nominee after consultation with Negroponte, Hayden's current boss. Outgoing CIA Director Porter Goss announced his retirement earlier this month after disputes with Hayden and Negroponte about the CIA's direction.

Some have questioned whether it is appropriate to have someone like Hayden, with his lengthy resume in military intelligence, directing the civilian spies at the CIA at a time when the intelligence community is increasingly dominated by the Pentagon.

Levin prefaced his questions by noting press reports about disagreements between Hayden and Rumsfeld over intelligence reform legislation that created the office of the Director of National Intelligence.

"Some people say you're just going be the instrument of the defense secretary. And if those reports are right, this would be an example where you disagreed with the defense secretary. After all, you wear a uniform and he is the secretary of defense. Are those reports accurate?" Levin asked.

Hayden downplayed the reports, adding that Rumsfeld "treated me with respect."

"Is it fair to say that on some of those issues there were differences between you and Secretary Rumsfeld?" Levin asked.

"Yes, sir," Hayden replied.

In closed door meetings with senators, Hayden, 61, indicated a willingness to retire from the Air Force if necessary to get the job. But during the hearing he was more cryptic.

"The fact that I have to decide what tie to put on in the morning doesn't change who I am," he responded to a question about whether he would retire. But, he added if his uniform "gets in the way" of his ability to bond the CIA together, "I'll make the right decision."

In an unusual move, the Senate was considering reconfirming Hayden as a four-star general in his new capacity at the CIA.

Current military law only allows active duty officers to keep their three and four star rankings while serving in military-related positions. The CIA job does not have that ranking though Hayden's current post as deputy director of national intelligence does.

Senate Armed Services Chairman John Warner, R-Va., and Levin, that committee's ranking member, have also included language in this year's defense authorization bill that would allow Hayden to keep his four-star rank while he serves as CIA director. The defense authorization bill would require that officers serving in civilian roles "not be subject to supervision or control by the secretary of defense or by any officer or employee of the Department of Defense, except as directed by the secretary or the secretary's designee concerning reassignment from such position."

On the question of the treatment of detainees, Hayden said he would address more of those questions in a closed session scheduled for Thursday afternoon, but he pledged adherence to the law.

"Obviously, we're going to follow the law. We're going to respect all the American international responsibilities," Hayden said.

FOX News' Brian Wilson, Sharon Kehnemui Liss and Gregory Simmons and The Associated Press contributed to this report.