( Reuters ) - As she struggles to blunt Barack Obama's growing momentum, Democrat Hillary Clinton shuffled her campaign staff's leadership on Sunday ahead of this week's U.S. presidential nominating contests, while her rival celebrated a victory in the Maine caucuses.
Clinton replaced her campaign manager after a string of losses on Saturday, though aides played down any notion the move was a sign of trouble for the New York senator in her tight state-by-state fight with Obama to be the Democratic nominee for the November 4 election.
Obama, an Illinois senator who would be the first black president, won in Maine after easy victories on Saturday in Louisiana, Nebraska and Washington state, and is rolling confidently into Tuesday's so-called Potomac primaries in Washington D.C, Maryland and Virginia.
John McCain, an Arizona senator who became the likely Republican nominee last week when his chief rival dropped out, lost two of three state contests on Saturday but got a vote of confidence from President George W. Bush on Sunday.
Clinton, who would be the first woman president, said in a statement that Maggie Williams, a top aide when she was first lady, had taken over from Patti Solis Doyle as her campaign manager. Solis Doyle will move into the role of senior adviser.
Clinton did not spell out why Solis Doyle was being replaced and a Clinton spokesman said the move did not reflect any change in the candidate's overall strategy.
But Larry Sabato, a political science professor at the University of Virginia, said the shake-up "can't be a good sign."
He said replacement of Solis Doyle along with Clinton's acknowledgment last week that she had made a personal loan to her campaign of $5 million were indications the candidate and her aides are concerned about the direction of the campaign.
"It indicates that they understand that things have not gone as well as they had hoped because if they had, the campaign would've been wrapped up by February 5," Sabato said.
Obama's landslide victory in South Carolina seemed to be a turning point in the race.
Sabato said that while Clinton may be in a bit of trouble, "it's not over," adding that if she does well in Ohio, Texas and Pennsylvania, she could nail down the nomination.
Former Arkansas Gov. Mike Huckabee beat McCain on Saturday in Louisiana and Kansas and ran a very close second in Washington state. Rep. Ron Paul of Texas, far behind in the Republican race, was a close third in Washington.
"It was great day for us," Huckabee said on Sunday on NBC's Meet the Press. "This thing could go to the convention."
While saying he was staying in the race, Huckabee added that he would bow out if McCain won the 1,191 delegates needed to capture the Republican nomination at the party's national convention in September.
McCain has grabbed just less than two-thirds of that number of delegates with more than 20 of the 50 states still to hold Republican nominating contests.
Bush, a Republican, has refused to endorse a candidate while Huckabee is still in the race. On Sunday, the president tried to calm fears among conservatives that McCain was too liberal by calling him a "true conservative."
"If John's the nominee, he has got some convincing to do to convince people that he is a solid conservative," Bush said on Fox News Sunday. "And I'll be glad to help him if he's the nominee, because he is a conservative."
With his victory in Maine, Obama was projected to pick up 15 delegates compared to nine for Clinton, his campaign said. The win gave him a clean sweep in the states that have held contests since "Super Tuesday" last week.
"I want to thank the people of Maine," Obama told a crowd of some 18,000 in Virginia Beach, where he was campaigning ahead of Tuesday's vote in a state where he has polled well.
Campaigning in Manassas, Virginia, on Sunday, Clinton said she was ready to go on Day One, blamed Bush for a series of problems and linked herself with popular Democratic President Harry Truman, who served from 1945 to 1953.
"I had a historian tell me the other day that it's probably not been since Harry Truman that we had a president who inherits two wars, an economy in trouble, millions of people losing their health care, millions of families on the brink of losing their homes," she said.
Clinton and Obama are about even in pledged delegates but both are well short of the 2,025 needed to win the Democratic nomination.
McCain virtually clinched the race on Thursday with the withdrawal of former Massachusetts Gov. Mitt Romney. But many conservative Republicans are unhappy with the prospect because of McCain's past voting record on such issues as taxes, immigration, stem cell research and campaign law reform.