Since opening his presidential bid 14 months ago, Senator Barack Obama of Illinois has answered many questions about his candidacy.
Can he turn inspiration into votes? Yes. Can he raise money? Yes. Can his organization compete with the political muscle of one of the best-known families in Democratic politics? Yes.
But after his defeats this week at the hands of Senator Hillary Rodham Clinton, there is frustration and anger among his supporters, advisers and contributors about the Clinton campaign’s attacks on him — and still-unresolved tension about how far he can go in striking back without sacrificing his claim to be practicing a new brand of politics.
The conflict was given new life on Friday when Samantha Power, a close friend and a senior foreign policy adviser to Mr. Obama, resigned after referring to Mrs. Clinton, of New York, as “a monster.”
While Ms. Power, a Pulitzer Prize-winning writer, apologized for remarks she called “inexcusable,” the incident underscored the hard feelings that have developed over a long primary race that is probably months from ending.
Mr. Obama, who did not publicly acknowledge Ms. Power’s comment when he arrived here Friday on the eve of the Democratic caucuses in Wyoming, privately admonished members of his staff to avoid being drawn into an unnecessary negative back-and-forth with rivals.
Asked about the incident by a reporter at a campaign stop here, he said he had not “been drawn into a knife fight.”
Yet after losing in Ohio, Rhode Island and Texas — following days of being pummeled — advisers to Mr. Obama conceded they had to take a sharper tack as the Democratic nominating fight slogs forward in a delegate-by-delegate battle.
While still calibrating how to challenge Mrs. Clinton, advisers to Mr. Obama seemed to settle on an approach by seizing on her delay in releasing income tax returns and the slow pace at which papers from the Clinton White House are being made public.
The Obama campaign manager, David Plouffe, told reporters that Mrs. Clinton was “one of the most secretive politicians in America today.”
The Clinton campaign has, since Bill Clinton ran for president, mastered the art of the “war room.” And this week, suddenly smelling blood, it seemed on full alert. On Friday alone, Mrs. Clinton and her aides held forth with telephone calls and news conferences hitting Mr. Obama on Iraq, Ms. Power and trade.
Even as they counterpunched, Mr. Obama’s aides cast themselves as reluctant participants in the brawl.
“There are people that will always do politics as usual better than we will,” said Robert Gibbs, the communications director for Mr. Obama. “That’s why people want something different.”
The comments from Ms. Power came in an interview with a Scottish newspaper in which she characterized Mrs. Clinton as a desperate candidate. “She is a monster, too — that is off the record — she is stooping to anything,” Ms. Power was quoted as saying.
While the comments were unauthorized and immediately condemned, they also drew attention to other remarks Ms. Power made in an interview with the British Broadcasting Corporation, saying that as president, Mr. Obama would not necessarily follow through on the plan of withdrawing from Iraq that he had presented as a candidate.
It is the second time in two weeks that the actions of a top aide have forced Mr. Obama to defend the idea that he means what he says — hardly the ideal situation for a candidate who asks voters to trust his judgment and integrity.
“While Senator Obama campaigns on his plan to end the war, his top advisers tell people abroad that he will not rely on his own plan should he become president,” Mrs. Clinton told reporters on Friday in Mississippi. “This is the latest example of promising the American people one thing on the campaign trail and telling people in other countries another.”
Here in Wyoming, Mr. Obama forcefully defended his stance on the Iraq war, reminding voters that he had opposed it from the beginning.
“I just have to mention this because I don’t want anybody here to be confused,” Mr. Obama said. “I was opposed to this war in 2002. If it had been up to me we would have never been in this war. It was because of George Bush with an assist from Hillary Clinton and John McCain that we entered into this war.”
Under Mr. Obama’s rules, the attack does not violate his pledge to wage a new type of politics because it revolves around a specific issue, like Iraq. At another point during a town-hall-style meeting here, he resorted to humor when he invoked the Clinton television advertisement of the telephone ringing in the White House at 3 a.m.
“What do people think I’m going to do? I’m going to answer the phone,” he said, speaking over laughter from the crowd. “I’m going to find out what’s going on.”
At a rally Friday evening in Laramie, Wyo., he hailed Mrs. Clinton as “a fine public servant.” He said her approach in Washington was not to unify but “to beat the other side into submission.”
Still, in what the campaign hoped would be the final days of the primary fight, Mr. Obama continues to grapple with some of the same challenges he faced in the opening weeks of the campaign. It was a year ago when Mr. Obama rebuked his staff for how they responded when David Geffen, a Hollywood executive and former fund-raiser to the Clintons, sharply criticized Mrs. Clinton.
“I told my staff that I don’t want us to be a party to these kinds of distractions because I want to make sure that we’re spending time talking about issues,” Mr. Obama said in an interview at the time. “My preference going forward is that we have to be careful not to slip into playing the game as it customarily is played.”
But with the stakes of the nominating battle high, some of Mr. Obama’s supporters worry that his campaign has not found a way to deal strongly with criticism from Mrs. Clinton. On Thursday, her senior advisers accused Mr. Obama of acting like the former prosecutor Kenneth W. Starr for raising questions about releasing papers from her time in the White House.
Charles Lundgren drove 320 miles from Montana to see Mr. Obama here. He said he had grown worried in recent days about the back-and-forth between the Democratic rivals.
“He has to come out and reveal some of her negatives more,” said Mr. Lundgren, 80. “Negatives seem to pay off; they apparently did in Texas. I think he’s going to have to come back at her. You can’t let Bill and Hillary contribute that way with their negative stuff without coming back at them.”
Michael Powell contributed reporting from New York.