Barack Obama easily won three more Democratic nominating contests on Tuesday, extending his winning streak over rival Hillary Clinton and building momentum in a hard-fought US presidential race.

Obama rolled to decisive victories in Virginia, Maryland and the District of Columbia, running his hot streak to eight consecutive wins and expanding his lead in pledged convention delegates who select the party's nominee.

"Tonight, we're on our way. But we know how much further we have to go," Obama told supporters in Madison, Wisconsin, where the next showdown occurs in a week. "We know our road will not be easy. But we also know that at this moment the cynics can no longer say our hope is false."    

Republican front-runner John McCain also swept to wins over his last major challenger, former Arkansas Gov. Mike Huckabee, in the US capital area contests as he moved closer to clinching the party's nomination for the November election.

McCain already was looking forward to a general election match-up with the Democrats after Tuesday's wins, which will increase pressure on Huckabee to give up his White House quest.

"Now my friends comes the hard part, and for America, the much bigger decision," McCain, an Arizona senator, told supporters in Alexandria, Virginia.

"We do not know for certain who will have the honour of being the Democratic Party's nominee for president. But we know where either of their candidates will lead this country, and we dare not let them," he said.

The wins for Obama, who would be the first black U.S.
president, followed big weekend triumphs in Maine, Louisiana, Nebraska, Washington and the Virgin Islands.

All three of Tuesday's contests occurred in fertile territory for Obama, with large populations of the highly educated, high-income and black voters who have favored the Illinois senator.

But exit polls indicated Obama dramatically expanded his support and cut into Clinton's core groups. Obama led among women, Hispanics and lower-income voters in Virginia and essentially split the votes of whites with Clinton. In Maryland, he captured seniors and rural voters.

"This is the new American majority," Obama said. "This is what change looks like when it happens from the bottom up."    

Obama already had edged past Clinton in the race for pledged delegates who formally select a party nominee at a convention in August. A total of 168 Democratic delegates were at stake in Tuesday's voting.

Obama had 1,017 pledged delegates to Clinton's 942, according to a count by MSNBC -- well short of the 2,025 needed to clinch the Democratic nomination.

Among Republicans, McCain has built a nearly insurmountable lead in delegates to the party's nominating convention and became the likely nominee last week with the withdrawal of top rival former Massachusetts Gov. Mitt Romney.

McCain had won 785 of the 1,191 delegates needed for nomination -- an overwhelming lead on Huckabee, who had 240.

But Huckabee won two of three contests on Saturday as McCain, an Arizona senator, struggled to win over disgruntled conservatives unhappy with his record on immigration, taxes and other issues.
    "We march on," Huckabee said on Fox News Channel. "We live to fight another day."    

Clinton, a New York senator and wife of former President Bill Clinton, has voiced confidence about her campaign's future but looked past Tuesday's contests and next week's battles in Wisconsin and Hawaii to focus on crucial March 4 votes in the big states of Texas and Ohio.

"We're going to sweep across Texas in the next three weeks," Clinton said in El Paso, Texas, where she headed on Tuesday before the day's results were known. She made no mention of the three contests she lost.

"I'm tested, I'm ready, let's make it happen," she said. Clinton wa