By John Whitesides, Political Correspondent

WASHINGTON (Reuters) - Democrat Barack Obama hoped to extend his winning streak over Hillary Clinton in their bruising Democratic presidential race when voters made their choices in three battlegrounds around the U.S. capital on Tuesday.

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 tried to move closer to clinching the party's nomination.

Voting took place in freezing temperatures with possible light snowfalls or rain forecast for the afternoon but analysts expected turnout to be strong.

Polls were to close at 7 p.m. EST/2400 GMT in Virginia and at 8 p.m. EST/0100 GMT on Wednesday in Maryland and the District, and results were expected soon afterward.

In the Democratic race, the Clinton camp conceded that Obama was favored in all three contests. Obama, who would be the first black U.S. president, holds a small lead in the state-by-state competition to represent the party in the November election.

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 formally select a party nominee at a convention in August.

Among Republicans, McCain has built a nearly insurmountable lead in delegates to his party's 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.

In an interview on WTOP radio on Tuesday morning, Huckabee was asked if he thought he might have a chance to win in Virginia. "We'd love to," the ordained Baptist minister said. "It depends on if our voters truly are willing to come out and show that they believe that there ought to be a real race."

All four candidates had 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. 168 delegates are at stake in Tuesday's voting.

Clinton, a New York senator and wife of former President Bill Clinton, 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 favor Obama -- to focus on crucial March 4 contests in the big states of Texas and Ohio.

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 on Monday. "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.

The former Navy pilot and Vietnam prisoner of war 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.

(Additional reporting by Deborah Charles, David Morgan, Caren Bohan, Jeff Mason, Andy Sullivan; Editing by David Storey)

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http://blogs.reuters.com/trail08/)