Obama easily swept four weekend contests in the states of Maine, Louisiana, Nebraska and Washington, edging past Clinton in the race for pledged delegates who select a party nominee.
Among Republicans, McCain has built a nearly insurmountable lead in delegates to the nominating convention and became the likely nominee last week with the withdrawal of his top rival, Mitt Romney, the former Massachusetts governor.
But Huckabee captured two of three contests on Saturday as McCain, an Arizona senator, struggled to win over disgruntled conservatives unhappy with his views on immigration and taxation in particular.
Polls close at 7pm (2400 GMT) in Virginia and at 8pm (0100 GMT) on Wednesday in Maryland and the District, with results expected soon afterward.
All four candidates criss-crossed the area around the nation's capital on Monday, hunting for support in a hard-fought presidential race where momentum has been difficult to sustain.
Obama, an Illinois senator, has 943 pledged delegates to Clinton's 895, according to a count by MSNBC - well short of the 2,025 needed to clinch the Democratic nomination.
Clinton, a New York senator, voiced confidence about her campaign's future even as she looked past the three contests on Tuesday and next week's battles in Wisconsin and Hawaii - where opinon polls have favoured Obama - to focus on crucial March 4 contests in the big states of Texas and Ohio.
"I am absolutely looking to Ohio and Texas because we know that those are states where they represent the broad electorate in this country," Clinton said.
"They represent the kind of voters that will have to be convinced and won over in the general election."
Clinton said she had the best chance of beating McCain, who has all but clinched the nomination by winning more than 700 of the 1,191 delegates needed for nomination - an overwhelming lead on Huckabee, who has barely more than 200.
|Huckabee captured two of three contests |
on Saturday [AFP]
Mark Penn, Clinton's strategist, said the former first lady would be better able to withstand Republican attacks in a general election, having faced them for years.
Clinton echoed the theme in an interview with a local Washington television station.
"I have been vetted, I have been through this. There isn't any new information," Clinton said. "I don't think you can say that about my opponent."
Obama, in a later interview with the same station, said he did not buy the argument. "What we have shown is that we can take a punch," he said. "We have shown we can take a loss."