With Senator Clinton racking up a string of recent defeats in the Democratic presidential contest, her campaign manager is departing and being replaced by an aide who served as Mrs. Clinton’s chief of staff during her years as first lady in the White House.

The installation of the campaign’s new chief, Margaret Williams, and the departure of the former head, Patti Solis Doyle, came as Mrs. Clinton’s rival for the nomination, Senator Obama of Illinois, yesterday swept caucuses in the state of Maine. Mr. Obama won 59% of the delegates to the state convention that will select national delegates, while Mrs. Clinton had 41%, with 91% of the precincts reporting. On Saturday, Mr. Obama also prevailed by healthy margins at caucuses in Nebraska, Louisiana, Washington, and the Virgin Islands.

In an e-mail message to colleagues, Ms. Solis Doyle, who moved to Iowa last year to oversee Mrs. Clinton’s ultimately unsuccessful effort there, alluded to some exhaustion but made no reference to the campaign’s difficulties or the challenges it faces in the coming weeks.

“I have been proud to manage this campaign, and prouder still to call Hillary my friend for more than sixteen years. I know that she will make a great President,” Ms. Solis Doyle wrote. “This has already been the longest Presidential campaign in the history of our nation, and one that has required enormous sacrifices from all of us and our families.” A campaign spokesman said the aide left of her own accord.

Mrs. Clinton issued a statement praising Ms. Solis Doyle “for an extraordinary job” that put the nomination “within reach.” The former first lady said she was “lucky” to have Ms. Williams taking the helm. “I know she will lead our campaign with great skill towards the nomination,” the New York senator said.

Beginning on Super Tuesday last week, when the two top Democrats essentially tied in a 22-state showdown, Mrs. Clinton’s aides have acknowledged that Mr. Obama was likely to score a series of wins until March 4, when the Clinton campaign believes it can prevail in Ohio and Texas. However, Mrs. Clinton’s team may not have expected that Mr. Obama’s margins in the February contests could push him out front in the delegate race.

During a conference call with reporters on Wednesday, Mrs. Clinton’s communications director, Howard Wolfson, seemed to predict that Mrs. Clinton would maintain her lead in the delegate count, even as Mr. Obama notched some victories this month.

“We think that we are in the pole position because we have a lead overall in delegates. We think it is going to be very difficult for Senator Obama to make up that lead because of the way in which the party allocates its delegates proportionally. So we feel very good about that, but this is going to be a neck and neck contest for the foreseeable future,” Mr. Wolfson told reporters. “Senator Obama does enjoy some advantages in the contests in the rest of February but not in a way that should permit to him to overcome our lead in delegates.”

By last night, however, Mrs. Clinton’s lead in the delegate race already seemed to have evaporated. The CBS News count had Mr. Obama at 1,134 delegates and Mrs. Clinton at 1,131, with 2,025 needed to secure the nomination under current rules. CNN, ABC News, and the Associated Press had Mrs. Clinton narrowly ahead, though it appeared the balance might shift in Mr. Obama’s favor after tomorrow’s primaries in Maryland, Virginia, and the nation’s capital.

In an e-mail last night, Mr. Wolfson said he had not intended to suggest that Mrs. Clinton would remain ahead in the delegate count through February, but only that she would emerge victorious. “I meant overall,” he said. “We will be the nominee.”

A professor of politics at the University of Virginia, Larry Sabato, said the Illinois senator’s showing, particularly in caucus states, has taken the Clinton camp by surprise. “They obviously underestimated how good a month Barack Obama was going to have. They’re going to have to try to put a stop to it,” the analyst said.

Mr. Sabato said Mrs. Clinton’s campaign needs a win in Wisconsin on February 19 to slow or reverse Mr. Obama’s momentum. “She will have to make a stand there. They can’t wait until March 4th and Texas and Ohio,” the professor said.