WASHINGTONL: Republican Mitt Romney notched a huge presidential primary triple win Tuesday, strengthening an already compelling case that he is the presumptive nominee in the battle for the White House.
Romney thumped main rival Rick Santorum in Maryland and the US capital Washington, and won a tighter but more important race in Wisconsin, US media projected, in a pivotal night for the party's frontrunner.
"Thank you to Wisconsin, Maryland and Washington, DC. We won them all! This really has been quite a night," Romney told euphoric supporters in Milwaukee, Wisconsin.
"We've won a great victory tonight in our campaign to restore the promise of America."
Impressive as the Romney victories were, Santorum insisted the Republican race to see who will challenge President Barack Obama in November was merely at "halftime," and that he would carry on at least into May, when there will be a series of primaries in states more favorable to his candidacy.
"The clock starts tonight," Santorum told supporters in his home state of Pennsylvania, which votes later in April.
"We've got three weeks to go out here in Pennsylvania and win this state. And after winning this state, the field looks a little different in May," he said.
But Santorum must feel the sting of a loss in Wisconsin, where he campaigned heavily over the last week. With nearly 90 percent of precincts reporting, Romney had 42 percent of the vote to Santorum's 38 percent.
Santorum virtually ignored the other two contests in the hope of snagging a victory in the Midwest, where the arch-conservative has fared far better than in the more moderate Northeast.
In Maryland, Romney was on 48 percent to Santorum's 29 percent, with former House speaker Newt Gingrich and Texas congressman Ron Paul far off the pace, according to partial results.
The trifecta puts Romney more than halfway to the magic number of 1,144 delegates needed to be crowned the Republican flag-bearer at the party's national convention in Tampa, Florida in August.
Tuesday appeared to mark the start of a new phase in Romney's campaign, as he turns his full attention to challenging Obama in November.
He is already acting like the nominee, training his political fire on Obama's "government-centered society" and no longer mentioning his Republican rivals while on the campaign trail.
Obama on Tuesday also essentially helped kick off a new phase in the election campaign, rebuking Romney by name in a speech and calling him to account for supporting what the Democratic president sees as a "radical" budget passed by congressional conservatives last week.
Obama accused Romney of championing cutthroat "social Darwinism" that neglects the middle class and favors the wealthy, and said the Republican candidate is seeking to institute such a budget on "day one of his presidency."
Romney launched a spirited retort, signaling his eagerness to square off with Obama.
"There's no question that under this president, this recovery has been the most tepid, the most weak, the most painful since the beginning of our recorded economic history," Romney said on the Sean Hannity radio show.
Romney has met stubborn skepticism from conservatives, who fear that the ex-governor of liberal Massachusetts will tack to the left once he wins the nomination in order to appeal to independents.
That scenario is fodder for Santorum, a harsh critic of abortion and gay rights who has tapped into conservative angst about the frontrunner.
But exit poll data from Maryland suggests Romney may be turning a corner with lower-income Americans and the most conservative of voters, people Santorum has counted on throughout the campaign.
Prominent Romney supporter Tim Pawlenty, a former Minnesota governor, said after Tuesday's primaries that while "it's not our place" to tell Santorum or Gingrich when to drop out, the nominations race "for all practical purposes is over."
"Let's face it. Mitt Romney is going to be the nominee," he said.
But Linda Fowler, a professor of government at Dartmouth College, expected the race to carry on nevertheless.
"It really turned out pretty predictably, and I don't think it changes anything," Fowler told AFP.
"With Pennsylvania a couple weeks away, I can't imagine Santorum wouldn't hang on to run in his home state."
Gingrich, too, was all in despite a miserable showing Tuesday, saying he was "committed to carrying the banner of bold conservative colors all the way to Tampa."
Romney has now won 24 out of 37 contests and amassed some 625 delegates of the 1,144 needed, according to a tally by website RealClearPolitics.
Santorum has racked up 11 victories and has well under half Romney's delegate count.