"Tonight, we're on our way. But we know how much further we have to go," Obama, a senator for Illinois, 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."
Barack Obama - 958
Hillary Clinton - 904
John McCain - 724
Mike Huckabee - 234
Icy roads in Maryland prompted a judge to order a 90-minute extension in voting hours, delaying returns and the allocation of delegates there.
In all, there were 168 delegates at stake in primaries in those states and the District of Columbia.
Clinton, Obama's Democrat rival, had seen Virginia as her best chance to cause an upset and the state was the biggest prize in Tuesday's elections with 83 Democratic delegates at stake.
Tuesday's primaries were considered relatively easy pickings for the Obama campaign, with all three having large populations of high-income and black voters who have backed Obama in other states.
Exit polls in Virginia indicated Obama had won about 50 per cent of white voters and crushed Clinton 9-to-1 among black voters.
Last weekend, Obama easily swept contests in the states of Maine, Louisiana, Nebraska and Washington, edging past Clinton in the race for pledged delegates who select the party's eventual nominee.
Clinton, a New York senator, held a rally in Texas on Tuesday night as results from the Washington-area primaries rolled in.
The Clinton camp is looking beyond February to the next primaries in Ohio and Texas on March 4.
Steve Livingston, a professor of political science at George Washington University, told Al Jazeera that Obama clearly had the momentum.
He said: "Obama is bringing in $1 million a day in donations - a remarkable figure. He is out-raising Hillary Clinton in the fund-raising race by a substantial margin."
"This is going to allow him to buy television advertising in Texas and Ohio that she [Clinton] cannot afford to match.
Despite the close Republican race in Virginia, McCain took all the delegates in the 'winner-take-all' poll, further adding to his huge lead over Huckabee.
However, Rob Reynolds, Al Jazeera's senior Washington correspondent, said the closeness of the Republican race in Virginia was a sign of the deep divisions within the party over McCain's candidacy.
Conservatives within the party have expressed concern at McCain's positions on immigration and tax cuts.
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.