Mrs Clinton insisted her historic quest was in good shape, despite a campaign staff shakeup and opinion polls that suggest she will tumble to defeat in the US capital, Maryland and Virginia after five Obama wins at the weekend.

'I think things have gone well. I think this is always going to be a competitive race because there's so much at stake,' the former first lady said in an interview with ABC's Washington affiliate on Monday.

'I've been around a long time, so, you know, sometimes you're up, sometimes you're down. I've been through all of that. But I feel really good about where we are.'

Clinton campaign operatives signalled they were now pinning their hopes on Ohio and Texas, big states which vote on March 4, to close an anticipated gap with the surging Obama.

Despite Mrs Clinton emerging from last week's 22-state Super Tuesday Democratic showdown neck-and-neck with the Illinois senator, he picked up weekend wins in Louisiana, Nebraska, Maine, Washington state, and the US Virgin Islands.

In so doing, Mr Obama nudged ahead in the race for delegates portioned out by each state, who will formally pick the party nominee at the Democratic convention in August.

Mr Obama now leads Mrs Clinton 1,144 to 1,138 in the running delegate count, according to website RealClearPolitics.com. A total of 2,025 delegates are needed for the nomination.

The role of some 440 still-undecided super-delegates - party luminaries who can choose to vote for either candidate - is now likely to be critical.

Polls opened in Virginia at 6am (1100 GMT), with Maryland and Washington DC set to follow an hour later, in the so-called Potomac Primary, named after the river that runs through all three jurisdictions.

Latest opinion surveys show Mr Obama poised for solid victories Tuesday, in a region with large numbers of African Americans and affluent white voters.

Virginia is the biggest prize of the Potomac Primary, with 83 Democratic delegates up for grabs, while Maryland has 70 on offer, and the US capital, which is not part of any state, 15.

On Monday, Mr Obama filled two arena-sized events with echoing cheers in Maryland, while Clinton campaigned in a quieter key, delivering a lecture at a university in southern Virginia and touring a General Motors plant in Maryland.

Mrs Clinton was asked in the ABC interview whether there were any hidden business or personal scandals stalking her husband, former president Bill Clinton, that Republicans could exploit.

'That is not going to happen. You know, none of us can predict the future, no matter who we are and what we're running for, but I'm very confident that that will not happen,' she said.

The question was one of the few occasions when the turmoil that wracked the Clinton White House has been directly raised in the 2008 campaign.

Mrs Clinton's replacement on Sunday of campaign manager Patti Solis Doyle with long-time aide Maggie Williams sparked a fresh spate of headlines about the former first lady's supposed vulnerability.

On the Republican side, Senator John McCain, who emerged from last week's Super Tuesday contests as presumptive nominee, enjoyed a more than 20 point lead in the latest polls in Maryland.

In an average of recent Virginia polls by RealClearPolitics.com, Sen McCain was up more than 20 points.

His last viable remaining rival, Mike Huckabee, vowed to fight on, despite having only a narrow chance of catching up.

'I think we've got a real shot of surprising people tomorrow,' Mr Huckabee told CNN late Monday, after dealing Sen McCain resounding defeats in two out of three races over the weekend.

'We have close to 800 delegates. Last time I checked, Governor Huckabee had very few, so I think I'm happy with the situation I'm in,' Sen McCain said Monday.

He needs 1,191 delegates to win the Republican presidential nod.-- AFP