U.S. Republican presidential hopeful Mitt Romney is moving closer to securing his party's nomination, after a sweep of Tuesday's primaries in Maryland, Washington, D.C. and Wisconsin. His closest challenger for the nomination, former U.S. senator Rick Santorum, has vowed to stay in the race and is hoping for a boost later this month from Pennsylvania, the state he represented in Congress.

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Former Massachusetts governor Romney celebrated his triple victories Tuesday night, before cheering supporters in Milwaukee, in the north-central state of Wisconsin.

"Thank you to Wisconsin, Maryland and Washington, D.C. We won 'em all! This really has been quite a night," Romney said. "We've won a great victory tonight in our campaign to restore the promise of America."

The wins follow high-profile endorsements for Romney, from Republicans who are calling on the party to unite behind him and focus attacks on President Obama, a Democrat.

Republican Delegate Count
With Tuesday's wins, the former governor has over 600 delegates - more than half the amount required for the nomination. Challenger Santorum has less than 300.

Former senator Santorum told supporters in Pennsylvania Tuesday the race is only midway through. He is hoping for a campaign boost in the state's April 24 primary.

"Ladies and gentlemen, Pennsylvania and half the other people in this country have yet to be heard," Santorum said.

Romney on Tuesday looked ahead to the election, taking swipes at the president, slamming his policies and blaming him for the country's economic woes and high gasoline prices.

President Obama had strong words for Republicans earlier Tuesday, telling the annual meeting of the Associated Press editors and publishers that the Republican budget plan is "an attempt to impose a radical vision on the country that would have devastating consequences for the middle class."

The president, who has seen high gas prices and economic concerns hurt his popularity, had an advantage over Romney in a national survey released Monday. The USA Today/Gallup poll indicated 49 percent of registered voters would vote for Obama if the election were held now, compared to the 45 percent who would support Romney.