Hillary Clinton remained defiantly upbeat about her presidential prospects yesterday despite a string of losses over the weekend to Barack Obama, who is expected to gain further momentum in the Potomac primaries today.

Her campaign has played down its prospects of winning any of the three contests today, when Mr Obama is expected to be buoyed once more by overwhelming backing from black voters, who account for a big proportion of the electorate in Virginia, Washington DC and Maryland.

Yesterday she acknowledged that many black voters face a challenging choice between her candidacy and Mr Obama's, adding: "It's a good problem to have." On Sunday she replaced her campaign manager, Patti Solis Doyle, with Maggie Williams, an African-American and long-time confidante who had been her chief of staff in the White House.

The latest shake-up reflects growing alarm within at Mr Obama's gathering momentum and apparent edge in fundraising. Yesterday Mrs Clinton dismissed claims that the appointment indicated her White House run was beginning to lose its way. Instead, she said: "There is just too much to be done, so we had to add some more people. There really is not significant change, we really just got to get more help." Mrs Clinton added that she was "absolutely looking forward to Ohio and Texas" - one of several references to states holding primaries on March 4.

Ms Williams, who once described her working philosophy as "Let them set the mission - let me get it done", has much to do in her new role if she is to salvage the presidential campaign of a woman she has served - on and off - for the best part of 25 years. By next month, Mr Obama could be heading into the March 4 primaries on the back of nine straight victories. One unaffiliated Democratic operative suggested that Mrs Clinton was sounding like Rudy Giuliani, who skipped early contests to concentrate on Florida only to see his formerly front-running campaign for the Republican nomination wilt away from the spotlight. Mrs Clinton argues that Ohio and Texas "are states that represent the broad electorate in this country, the kind of voters that are going to have to be convinced and won over in the general election".

She suggested that Mr Obama's multiple victories in caucus states - including many of those holding elections last weekend - were of far less value because they are "dominated by activists which don't represent the electorate". She even appeared derisive when discussing earlier contests such as heavily Republican Idaho, North Dakota and Alaska. "We're not going to win them ," Mrs Clinton said.

Asked to cite the differences between herself and this "shiny new toy Barack Obama", she replied: "Well, I'm battle scarred. And I'm proud of those scars."

Ms Williams has been at Mrs Clinton's side for many of those battles. She is said to "know everybody" and command huge respect within the Democratic Establishment - potentially an important factor as the battle for super-delegates becomes more intense. A well-read and intellectually combative black woman, Ms Williams may also restore some of the Clintons' tarnished lustre among African-Americans - who have favoured Mr Obama in recent elections.

Mr Obama suggested on Sunday that it would be "very hard for Senator Clinton to break out of the politics of the past 15 years", which were marked by bitter partisan divisions. She "starts off with 47 per cent of the country against her", Mr Obama told a rally of 3,000 people in Alexandria, Virginia: "that's a hard place to start".

Last night he was scheduled for talks with John Edwards, seeking a coveted endorsement from a former rival who dropped out of the race two weeks ago and still has about 40 convention delegates to his name. There are rumours that both candidates are discussing senior posts in their putative administrations, including that of Attorney-General, which would give Mr Edwards a swift return to the public eye.

In the Republican race, Mike Huckabee reiterated yesterday that he would not step aside for the likely nominee, John McCain, "as long as my guys are still waving the pompoms".