WASHINGTON — President Bush’s nominee to run the Pentagon would counsel against a military strike targeting Iran except as a last resort and says he does not think the Iranian regime would use nuclear weapons in a first strike against Israel.

In an all-day hearing before the Senate Armed Services Committee yesterday, the nominee to be defense secretary, Robert Gates, also said he would cooperate within his power with the incoming Democratic chairman’s nearly three-year investigation into intelligence analysis from the Office of Special Plans, a Pentagon unit established in 2002 to plan for the run up to the Iraq war that Democrats have accused of funneling bad intelligence to the White House.

The hearings yesterday bolstered the conventional wisdom that the appointment of Mr. Gates, a former member of the Iraq Study Group, signals a softening of the Bush administration’s approach to the wider war on terrorism. In response to a question from the incoming chairman, Senator Levin, a Democrat from Michigan, Mr. Gates said America was “not winning” the war in Iraq. He also said he agreed with the assessment of the chairman of the Joint Chiefs of Staff, General Peter Pace, who has said America is neither winning nor losing the war.

In response to a question from Senator Byrd, a Democrat of West Virginia, he said, “I think that military action against Iran would be an absolute last resort, that any problems that we have with Iran, our first option should be diplomacy and working with our allies to try and deal with the problems that Iran is posing to us.”

When asked, the president has said for more than a year that military strikes were options “on the table,” and that he would not allow Iran to acquire nuclear weapons during his tenure as president.

Mr. Gates, however, publicly emphasized the dangers of military action against Iran in a way that other administration officials have not. “I think that we have seen, in Iraq, that once war is unleashed, it becomes unpredictable. And I think that the consequences of a military conflict with Iran could be quite dramatic,” Mr. Gates told Senator Byrd. “And therefore, I would counsel against military action except as a last resort and if we felt our vital interests were threatened.” He also said that attacking Syria would be costly and “unleash a wave of anti-Americanism” in the region.

In another set of questions from Senator Graham, a Republican from South Carolina, Mr. Gates said he believed that Iran desired nuclear weapons and that Iran’s president was “not kidding” when he spoke of his desire to wipe Israel off the map. But Mr. Gates also cautioned that President Ahmadinejad did not have the final say in the regime and that a nuclear weapon would likely be acquired as a deterrent to attack from others. When pressed though, the nominee said, “If Iran obtains nuclear weapons no one can promise that it would not use them against Israel.”

For Israeli planners, however, this is precisely the problem. The Jewish state’s top security officials have said for the last two years Iran has emerged as the top paymaster of Palestinian Arab terrorism. A nuclear weapon, they reason, would only further embolden the Islamic Republic’s meddling.

Mr. Gates in the hearing also said he would assist Senator Levin “to the extent I have the authority,” in his quest for some 58 documents denied him up to now from the Pentagon in his investigation into intelligence analysis from the Pentagon’s Office of Special Plans. That office was overseen by the undersecretary of defense for policy, Douglas Feith, who retired from public service in 2005. Some Democrats have worried that their party will appear to be mired in the past if new Congressional chairman investigate claims about pre-war intelligence. Mr. Gates said he had not read that much about the office, but from what he did read, “I had a problem with it.”

On the topic of the Baker-Hamilton commission or Iraq Study Group on which he served until November 8, Mr. Gates was on the same page as the administration. He said that the suggestions from the panel were one set of insights that would be examined by the president and made reference to the administration’s internal Iraq policy reviews. The commission is scheduled to make its recommendations public today.

The Senate Armed Services Committee, which includes Senator Clinton, approved Mr. Gates’ nomination by a unanimous vote last night. The full Senate is expected to confirm him today.