Washington: Republican presidential front- runner Mitt Romney on Wednesday swept the three key primaries in Wisconsin, Maryland and Washington DC, cementing his status as the almost certain nominee of the party to challenge incumbent Barack Obama in the November polls.

"This has really been quite a night. We won a great victory tonight in our campaign to restore the promise of America," 65-year-old Romney, the former Massachusetts Governor, said in a victory speech to supporters in Milwaukee.

Romney's triple win puts him past the halfway mark to the 1,144 delegates needed to clinch the Republican nomination.

Romney has collected 648 delegates since the primary and caucuses began in January, more than twice the 264 delegates his nearest rival Rick Santorum is estimated to hold. Former US House of Representatives Speaker Newt Gingrich and Texas Congressman Ron Paul trailed far behind in primary results.

Santorum, the former Senator from Pennsylvania, who has faced calls to bow out of the race in the name of party unity, defiantly vowed to fight on and seemed to pin all his hopes on the primary in Pennsylvania which is scheduled for April 24.

"I ask you over the next three weeks, this isn't half-time, no marching bands. We're hitting the field. The clock starts tonight. 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," Santorum said in Pennsylvania.

In his speech, Romney also attacked the Obama administration and his inability to address the concerns of American people.

"We all know that President Obama cannot run on his record. We know that he can't run on his broken promises," he said.

"I seem to remember him saying that he was going to be a uniter, not a divider. Frankly I think this is one of the worst of his broken promises. We don't need a campaigner in chief. We need a commander in chief. We need a leader that America deserves," Romney said.

"The presidency is bigger than this. He was supposed to be bigger than this. We need solutions, not excuses. We need a President who takes the lead, not one who spreads the blame. We need someone who appeals to our dreams and our aspirations, not to our fears and our anxieties," Romney said.

If elected President, Romney vowed to spend the next four years rebuilding the foundation of a opportunity society led by free people and free enterprises.
"In Obama's government-centred society, the government has to do more because the economy is doomed to do less, because when you attack business and you vilify success, you are going to have less business and less success," he said.

"In Obama's government-centred society, government spending always increases because, well, why not? There's always someone who's entitled to something more and who's willing to vote for anyone who will give them something more," Romney said.

"I don't want to transform America. I want to restore to America the economic values of freedom and opportunity and limited government that has made us the powerhouse of the world," Romney said.

Following the three victories, American media was quick to pronounce that Romney has cemented his status as the almost certain Republican nominee and put new pressure on rival Santorum to significantly scale back his campaign.
"Mitt Romney tightened his grip on the Republican nomination with a sweep of the primaries in Wisconsin, Maryland and the District of Columbia, and found himself in his first direct engagement with President Obama, an unmistakable signal that the general election would not wait for internal Republican politics," the a daily said.

"Mitt Romney captured presidential primaries in Maryland, the District and battleground Wisconsin, the biggest prize of the day, to complete a momentum-building, three- contest sweep on Tuesday that cemented his status as the almost certain Republican nominee and put new pressure on rival Rick Santorum to reassess his candidacy," the Washington Post commented.