MARS, Pa. — Mitt Romney swept Tuesday's primaries in Maryland, Wisconsin and the District of Columbia, putting more distance between himself and the rest of the GOP presidential field in a race that party leaders have said they would like to see wrapped up soon.
Chief rival Rick Santorum was a distant runner-up in Maryland and significantly behind Mr. Romney in Wisconsin. He didn't even bother to file for the ballot in the District, an indication of the struggle faced by those seeking to deny Mr. Romney the nomination.
"We won 'em all," the former Massachusetts governor exulted at his victory party in Milwaukee. "This really has been quite a night. We won a great victory tonight in our campaign to restore the promise of America."
The night's wins officially gave Mr. Romney more than 600 delegates, which is more than halfway to the 1,144 needed to win the nomination at the Republican National Convention in Tampa, Fla., in late August.
Heading into Tuesday, the Associated Press tally showed Mr. Romney had collected 572 delegates, exactly half of the number needed. Mr. Santorum had 272, Mr. Gingrich had 135 and Mr. Paul had 51. While most of the political world was focused on the competition between Mr. Romney and Mr. Santorum for the 42 delegates up for grabs in Wisconsin, a total of 95 delegates were on the line.
Rep. Ron Paul of Texas and former House Speaker Newt Gingrich were battling for second in the District of Columbia and third in the other two states, continuing to search for momentum to keep them in the race.
But none of them showed any sign of conceding ahead of primaries on April 24 in Connecticut, Delaware, New York, Pennsylvania and Rhode Island.
Mr. Paul is looking past those states to California, and Mr. Gingrich has announced plans to campaign in Delaware. Mr. Santorum hopes to use his home state of Pennsylvania to rejuvenate his bid.
Speaking to more than 200 people gathered in a hotel ballroom in Mars, just north of Pittsburgh, Mr. Santorum said the race is now at halftime.
"Who is ready to charge out of the locker room in Pennsylvania for a strong second half?" he said, sparking applause from the crowd.
Mr. Santorum likened his never-say-die approach to Ronald Reagan's failed presidential bid in 1976 and the surprise attack that George Washington launched against Hessian forces when he crossed the Delaware River in 1776, which "turned the tide of the Revolution."
"Pennsylvania and half the other people in this country have yet to be heard, and we are going to go out and campaign here and across the nation to make sure their voices are heard in the next few months," he said.
A Quinnipiac University poll released Tuesday boosted his hopes, finding that Mr. Santorum holds a 41 percent to 35 percent lead in Pennsylvania over Mr. Romney with three weeks to go. Mr. Santorum served four terms in the House and two terms as a U.S. senator from Pennsylvania before losing his seat in the 2006 elections.
But with most states having already held primaries or caucuses, the chances are dimming for the others to catch up with Mr. Romney.
Mr. Romney increased his lead over the weekend even without another state voting, when the North Dakota Republican Party's state convention awarded him a majority of delegates from that state — even though Mr. Romney had placed third in caucuses there earlier this year.
Local reports said Mr. Romney and state party officials outmaneuvered Mr. Santorum and Mr. Paul, who placed first and second in the caucuses there on Super Tuesday last month.
In Tuesday's voting, Republicans again seemed to break between those who don't trust Mr. Romney, and those who think he's the best candidate the party has in a general election matchup with Mr. Obama.
"I think he has a better chance of winning than Santorum or Gingrich. Both those guys tend to shoot themselves in the foot," said Charles Robinson, 63, as he voted in Gaithersburg.
His wife, JoAnn, 62, a self-identified Democrat who plans to vote Republican in this year's election, said she preferred Mr. Gingrich and is wary of Mr. Romney.
"I just think that he switches a lot. He doesn't inspire confidence. I want to vote for somebody that I can trust, but I guess I have to vote for him," she said.
Mr. Santorum, who won Louisiana's primary late last month, was searching for another win to shield him from the growing number of Republicans who say it's time to end what has become a bloody contest and to rally behind Mr. Romney before it is too late.
Mr. Santorum's campaign sent an email fundraising plea Tuesday asking for money to help cover the costs of airing commercials in Pennsylvania.
"We can win Pennsylvania as long as we have the resources to compete. A win there will give us the momentum we need going into Texas, where the nomination may be decided," Nadine Maenza, Mr. Santorum's finance director, said in the email, which closed with "together we can help spread the truth about Mitt Romney and President Obama."
Mr. Obama, meanwhile, signaled during a speech to newspaper editors in Washington on Tuesday that he is transitioning into full campaign mode. He delivered a fierce attack on House Republicans' budget, which passed that chamber last week, as "thinly veiled social Darwinism," and chided Mr. Romney for supporting the plan.
"[Mr. Romney] said that he's 'very supportive' of this new budget, and he even called it 'marvelous' [-] which is a word you don't often hear when it comes to describing a budget," Mr. Obama said before slipping in a jab at the Republican front-runner's stiff style on the hustings. "It's a word you don't often hear generally."
Mr. Romney shot back in an interview with radio host Sean Hannity later in the day, saying the president was distorting the truth.
"I mean, this is a president who so misrepresents the policies and proposals of our party and of myself as well and then fails to acknowledge the mistakes and the errors in his own record," Mr. Romney said.
He added that he expects voters will conclude that "what he's saying about himself misrepresents the reality of his record."
Mr. Romney spent most of his victory speech laying out a vision to compete with Mr. Obama's plan for higher taxes to support the more expansive role he wants to see government play in spending on the social safety net.
"Under Barack Obama, America hasn't been working," he said. "The ironic tragedy is that the community organizer who wanted to help those hurt by a plant closing became the president on whose watch more jobs have been lost than any time since the Great Depression."
The three contests Tuesday were the first since Mr. Romney received high-profile endorsements from Sen. Marco Rubio of Florida, House Budget Committee Chairman Paul Ryan of Wisconsin and former President George H.W. Bush — all of whom argued that it was time to end the nomination fight and for the party to turn all of its attention to ousting Mr. Obama.
• David Hill contributed to this report.
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