The night's results increase the prospect of a contest next November between Obama and John McCain, who also won all three of his contests against Mike Huckabee.

Clinton now needs to win 56% of the remaining delegates if she is to secure the Democratic nomination. Her campaign showed new evidence of disarray tonight with the resignation of her deputy campaign manager, Michael Henry. As the results came in, her campaign worked the telephones in an effort to reassure donors that she remained a viable candidate.

Obama, speaking at Madison, Wisconsin, which holds its primary next week, told supporters he represented a "new American majority". He added:

"We won the state of Maryland. We won the commonwealth of Virginia and, though we won in Washington, DC, this movement will not stop until there is change in Washington, DC. Tonight we are on our way."

Clinton, at a rally in El Paso, Texas, did not refer directly to the elections. She mentioned a Texas putdown - all hat and no cattle. But it was an odd reference given that Obama has won more states than she has and more delegates.

McCain, in his victory speech in Virginia, was already setting his sights on Obama - and stealing his signature lines. "My friends I will not yield. I am fired up and ready to go," McCain said.

With nearly all of the votes counted, Obama had 63% against 36% for Clinton in Virginia - the one state where she had hoped to remain competitive. Early returns showed him on course for a convincing win in Maryland. In Washington, DC, Obama was leading Clinton by 76% to 24% with nearly all of the votes counted.

On the Republican side, McCain had an easy wins in Maryland and Washington, DC, where he had 67% of the vote against 17% for Huckabee with nearly all of the votes counted.

However, he had a much tougher fight than expected against Huckabee in Virginia. With nearly all the precincts reporting from Virginia, McCain was ahead by 50% over 41% for Huckabee.

The close finish in Virginia was a further sign of dissatisfaction with McCain from a core Republican constituency: evangelicals, who make up 40% of party supporters in the southern state.

With last night's results, Obama now has a string of eight consecutive victories over Clinton in the last week. He also has a measurable lead over Clinton in elected delegates.

That fresh burst of momentum threatens to overwhelm Clinton's plan of making a last stand in the mega states of Texas and Ohio on March 4 - especially given the new pressures on her to secure delegates.

Tonight's convincing wins for Obama also complicate the Clinton's fallback plan of winning a large share of super-delegates, senior party officials who are free agents in the Democratic contest.

The scale of Obama's victories, as well as signs that he is expanding his support into a more broadly based coalition, exposed growing problems for Clinton.

With tonight's performances, Obama began for the first time to bite into Clinton's support base.

Obama registered his strongest performance so far among white voters - without giving up his dominance among African-American voters.

He also for the first time began to make huge inroads into the constituencies Clinton could normally take for granted: white women and working-class households at the bottom of the economic ladder.

In the Republican race, the primaries confirmed the problems that McCain has with his party base. While he is still on course to secure - eventually - the Republican nomination, Huckabee's strong challenge in Virginia is a huge embarrassment for McCain. It comes after McCain's losses in two contests - Louisiana and Kansas - to Huckabee at the weekend.

In Virginia, about 46% of those voting in the Republican race described themselves as Christian evangelicals. Of those, about 70% voted for Huckabee, a Baptist pastor.

Many other Republicans voted for Huckabee as a protest against McCain, who is viewed as too moderate. McCain, the senator for Arizona, has angered Republicans because he voted against tax cuts introduced by president George Bush and because he has shown a willingness to work alongside Democrats in the Senate.

The Clinton team had been braced for tonight's defeats but the breakdown of the figures was worse than they could have anticipated.

The team was also shaken by another key resignation. In an email message Tuesday to staffers obtained by The Associated Press, Mike Henry said he was stepping down to allow campaign manager Maggie Williams to build her own staff. Williams replaced Patti Solis Doyle during the weekend. Solis Doyle had recruited Henry to join the campaign last year.

"Out of respect for Maggie and her new leadership team I thought it was the best thing to do," Henry wrote. "Our campaign needs to move quickly to build a new leadership team, support them and their decisions and make the necessary adjustments to achieve the winning outcome for which we have all worked so hard."

In a crucial blow to Clinton, exit polls suggested that she had failed to win over the blue-collar voters who had previously supported John Edwards before his withdrawal from the Democratic race last month. Those votes are now going to Obama instead.

Obama's coalition until tonight had been built round African Americans, and young people, as well as high-income and highly educated whites. Tonight, he matched her - and, in some cases, took a larger share of the vote - among white women and men and among the poor.