WASHINGTON — Barack Obama powered past Hillary Rodham Clinton in the race for Democratic convention delegates Tuesday on a night of triumph sweetened with outsize primary victories in Maryland, Virginia and the District of Columbia.

"Tonight we're on our way," Obama told cheering supporters in Madison, Wis. "But we know how much further we have to go," he added, celebrating eight straight victories over Clinton, the former first lady now struggling in a race she once commanded.

Late Tuesday, with more than 70 delegates still to be awarded, the Associated Press count of delegates showed Obama with 1,223. Clinton had 1,198, falling behind for the first time since the campaign began. Neither was close to the 2,025 needed to win the nomination.

His victories were by large margins — he was gaining about 75 percent of the vote in the nation's capital and nearly two-thirds in Virginia. In Maryland, he was winning close to 60 percent.

By contrast, Clinton was attempting to retool her campaign in the midst of a losing streak. Her deputy campaign manager resigned, the second high-level departure in as many days.

Campaigning in Texas, where she hopes to triumph on March 4, she said she was looking ahead, not back.

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

Republican front-runner John McCain won all three GOP primaries, adding to his insurmountable lead in delegates for the Republican nomination. He congratulated Mike Huckabee, his sole remaining major rival and a potential vice presidential running mate, then turned his focus on the Democrats.

"We know where either of their candidates will lead this country, and we dare not let them," he told supporters in Alexandria, Va. "They will paint a picture of the world in which America's mistakes are a greater threat to our security than the malevolent intentions of an enemy that despises us and our ideals."

Interviews with voters leaving the polls showed Obama narrowly defeated Clinton among white voters in Virginia, 52 percent to 47 percent, the first time he has done that in a Southern state and only the fourth time he has done so in a competitive primary this year. Clinton won the white vote by 10 percentage points in Maryland. He won 90 percent of the black vote in Virginia and almost as much in Maryland. She won a majority of white women in both states, though by less than she is accustomed to. He won among white men in Virginia, and they split that vote in Maryland.

In all, there were 168 Democratic delegates at stake Tuesday.

Obama moved past Clinton in the delegate chase on the basis of the day's primaries and newly released results from last Saturday's Washington caucuses. Additional delegates still to be allocated from his new victories were certain to add to his lead.