Santorum Faces Uphill Battle as Romney Wins 3 More Primaries

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(CNN) -- Rick Santorum turns his focus to his home state of Pennsylvania on Wednesday as he faces an uphill battle to convince Republicans he can stop Mitt Romney from clinching the party nomination.

Romney won Tuesday's Republican presidential primaries in Wisconsin, Maryland and the District of Columbia, according to unofficial numbers.

His three-contest sweep solidifies his lead and widens the gap with Santorum, who pledged to press on to his home state of Pennsylvania and beyond despite the losses. He is scheduled to attend events in the state Wednesday.

Romney's wins put him past the halfway mark to the 1,144 delegates needed to clinch the nomination, according to CNN estimates.

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

He made no mention of his GOP rivals and mstly contrasted himself with President Barack Obama.

"There is a basic choice that we're going to face: The president has pledged to transform America, and he spent the last four years laying the foundation for a new government-centered society. I will spend the next four years rebuilding the foundation of an opportunity society led by free people and free enterprises."

The former Massachusetts governor is expected to take the majority of Wisconsin's 42 delegates and Maryland's 37 delegates, with some designated solely for the winner and the rest to be awarded proportionally. He will take all 16 delegates at stake in the District of Columbia.

Counting partial allocations for Wisconsin and Maryland, and full allocations for D.C., Romney has collected 648 delegates since the primary and caucuses began in January, according to CNN estimates. That's more than twice the 264 delegates Santorum is estimated to hold. Gingrich and Paul trailed well back.

Despite the setback, Santorum vowed to press on Tuesday night to victory in the Republican nomination battle.

"Pennsylvania and half of the other people in this country have yet to be heard," he said. " And we're 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.

After Tuesday, five more states vote on April 24.

New York, Connecticut, Rhode Island and Delaware are leaning toward Romney. The fifth, Pennsylvania, is Santorum's home state, but polls show his once-large lead there has disappeared.

Romney led the pack by a wide margin in Tuesday's contests.

In the District of Columbia, with 100% of the vote reported, Romney had 68%. Ron Paul had 12% and Newt Gingrich had 10%, according to unofficial numbers on the state board of election website. Santorum was not on the D.C. ballot.

Romney received 48% of the vote in Maryland, with about two-thirds of the precincts reporting on the State Board of Elections website. Santorum notched 30%.

In Wisconsin, Romney had more than twice as many votes as Santorum in Ozaukee County, with 23% of the votes counted. He also led in the city of Milwaukee and Racine County.

Obama, meanwhile, clinched the Democratic presidential nomination Tuesday with primary wins in Maryland and the District of Columbia. Unlike the Republicans, Obama faces no serious opposition in his race.

According to CNN's delegate estimate, the president had 2,735 of the 2,778 delegates needed to secure his party's nod before Tuesday's contest. He is expected to win most or all of the 119 delegates at stake in Maryland and D.C., as well as the 100 delegates at stake in Wisconsin.

Speaking at a restaurant in Waukesha, Wisconsin, Romney suggested Obama wants to duck responsibility for the country's setbacks.

"If I am president, I will not only get things right again, I will take full responsibility for my errors and make sure that people understand we have a president in the White House again where the buck will stop at his desk," Romney said.

Later Tuesday, Obama mentioned Romney by name in a speech for the first time this year, while slamming a House-passed budget proposal that Romney has embraced.

Obama said the plan, which would lower tax rates and cut spending while reforming the Medicare and Medicaid government-run health care programs, was "thinly veiled Social Darwinism" and "antithetical to our entire history as a land of opportunity and upward mobility for everyone who's willing to work for it."

"One of my political opponents, Gov. Romney, has said that he hoped a similar version of this plan from last year would be introduced on day one of his presidency," Obama said. "He 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."

Romney fired back in an interview with Milwaukee radio station WTMJ.

Obama is engaging "in the most incendiary rhetoric he can find out of an attempt to ignore the fact that we can't keep on spending a trillion dollars more than we take in every year," Romney said.

CNN's Tom Cohen contributed to this report.