Sen. Barack Obama won the Democratic primaries in Virginia, Maryland, and the District of Columbia on Tuesday.
It was his eighth straight triumph over Clinton, the former first lady, now struggling in a race she once commanded.
"Tonight we're on our way," he told cheering supporters in Madison, Wis. "But we know how much further we have to go on," Obama added.
His Virginia victory brought him only 10 delegates short of his rival, with 132 left to allocate for the night.
Interviews with Virginia voters leaving the polls showed Obama split the white vote with Clinton, and his share of the black vote approached 90 percent. She led among white women, but he was preferred by a majority of white men.
Icy roads in Maryland had prompted a judge to order a 90-minute extension in voting hours, delaying returns and the allocation of delegates there.
Clinton Brushes off Obama Victories
But Clinton brushed off her recent losses to Obama. Addressing supporters in El Paso, Texas, the New York senator vowed to campaign across the delegate-rich state over the next three weeks to remind voters of her experience.
"I'm tested, I'm ready. Now let's make it happen," she said.
Clinton has been quick to point to what she calls Obama's lack of experience, and again insisted she is the candidate that would be ready to lead "on day one."
Earlier today, Clinton's deputy campaign manager resigned. That move marks the latest departure in a staff shake-up following the string of losses to Obama.
Maggie Williams replaced Patti Solis Doyle over the weekend as Clinton's campaign manager.
The Race for Delegates
Overall, Clinton had 1,164 delegates to 1,154 for Obama. Both are far from the 2,025 needed to win the nomination at the Democratic National Convention this summer.
Since last week's Super Tuesday contests in 22 states, Obama has won a primary in Louisiana as well as caucuses in Nebraska, Washington and Maine, all of them by large margins.
Obama has campaigned before huge crowds in recent days, and far outspent his rival on TV advertising in the states participating in the regional primary in Maryland, Virginia and the District of Columbia.
He began airing commercials in the region more than a week ago, and spent an estimated $1.4 million. Clinton began hers last Friday, at a cost estimated at $210,000.
With Clinton facing a series of possible defeats, and Obama riding a wave of momentum, the two camps debated which contender is more likely to defeat McCain in the general election.
An Associated Press-Ipsos poll found Obama with a narrow lead over the Arizona senator in a potential match-up, and Clinton running about even.
Source: The Associated Press