Democrat Barack Obama hopes to extend his winning streak over rival Hillary Clinton on Tuesday, when voters in three battlegrounds make their choices in a close and bruising Democratic presidential race.

Republican front-runner John McCain and his last major challenger, former Arkansas Gov. Mike Huckabee, also square off in Virginia, Maryland and the District of Columbia as McCain tries to move closer to clinching the party's nomination.

In the Democratic race, Obama is favoured in all three contests as he and Clinton battle for the 168 pledged convention delegates at stake in the voting.

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, former Massachusetts Gov. Mitt Romney.

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, taxes and other issues.

Polls close at 7 p.m. EST/Midnight British time in Virginia and at 8 p.m. EST/1 a.m. British time on Wednesday in Maryland and the District, with results expected soon afterward.

All four candidates crisscrossed 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 - all of which favour 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."

LOOKING TO NOVEMBER

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.

Clinton strategist Mark Penn said in a memo the former U.S. 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."

McCain said on Monday he would not take public matching funds in the nominating fight, avoiding their accompanying spending limits and allowing him to raise and spend more money before the nominating convention.

He still faces opposition in his own party from conservatives unhappy with his views on immigration and other issues, but he has promised to mend fences with them.

At a Monday night rally in Richmond, he was joined by two conservative former Virginia governors - James Gilmore and George Allen - and Christian conservative leader Gary Bauer, who endorsed McCain.

He promised he would appoint conservative judges when he is president and touted his support for recently appointed Supreme Court justices Samuel Alito and Chief Justice John Roberts, both popular with conservatives.

"I will appoint and nominate people who only have a clear and complete adherence to the Constitution of the United States and do not legislate from the bench," the former Navy fighter pilot and Vietnam prisoner of war said.