Mitt Romney and Rick Santorum

Mitt Romney and Rick Santorum (CNN)

(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 after his three-primary sweep.

Romney's wins in Wisconsin, Maryland and the District of Columbia put him past the halfway mark to the 1,144 delegates needed to clinch the nomination and add to a wide lead over other GOP presidential candidates, according to CNN estimates.

Yet Santorum indicated Tuesday night he would compete in the April 24 primaries in five states, including Pennsylvania, where his campaign hopes a win would be a gateway to a run of May primaries in states where he can capitalize on higher percentages of conservative voters.

Romney, in a speech to supporters in Milwaukee, made no mention of his GOP rivals and instead mostly contrasted himself with President Barack Obama.

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

He continued his criticism of Obama's economic polices and what he repeatedly called Obama's plan for a "government centered society."

"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.

With 93% of the vote reported in Wisconsin, Romney had 42% and Santorum, a former U.S. senator from Pennsylvania, had 38%. U.S. Rep. Ron Paul of Texas had 12%, and former House Speaker Newt Gingrich had 6%.

In Maryland, with 75% of the vote reported, Romney had 49%, Santorum had 29%, Gingrich had 11% and Paul had 10%.

In the District of Columbia, with 99% of the vote reported, Romney had 70%, Paul had 12% and Gingrich had 11%. Santorum was not on the D.C. ballot.

Pre-primary polls appeared to show Wisconsin's contest was the only one Tuesday that Santorum had a chance to win. And analysts said Wisconsin might be Santorum's final chance to slow Romney's march toward the GOP nomination.

But Santorum told supporters in Mars, Pennsylvania, on Tuesday night that the race was essentially at halftime, with only roughly half the available delegates awarded.

"Pennsylvania and half the other people in this country have yet to be heard, and we're going to go out and campaign here and across this nation to make sure that their voices are heard in the next few months," Santorum said.

Santorum, who has touted himself as a more conservative candidate than Romney, also warned supporters that the Republican Party often has fielded moderate Republicans against incumbent Democratic presidents, only to see the moderate Republican lose. He appeared to raise Ronald Reagan, a conservative who defeated Jimmy Carter in 1980, as an example for a way forward.

"If we're going to win this race, we can't have little differences between our nominee and President Obama. We have to have clear, contrasting colors," Santorum said. "... Time and time again, the Republican establishment and aristocracy have shoved down the throats of the Republican Party and people across this country moderate Republicans because, of course, we have to win by getting people in the middle. There's one person who understood we don't win by moving to the middle. We win by getting people to the middle to move to us and move this country forward."

President Barack Obama, meanwhile, clinched the Democratic presidential nomination Tuesday with primary wins in Maryland, the District of Columbia and Wisconsin, CNN projected. 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.