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 Tuesday that cemented his status as the almost certain Republican nominee and put new pressure on rival Rick Santorum to reassess his candidacy.

With his campaign increasingly focused on President Obama and the general election, the former Massachusetts governor’s victories in Maryland and the District were never in doubt. He won both by crushing margins. In Wisconsin, where Romney and Santorum devoted most of their energies, the margin was narrower but nonetheless decisive.

With Tuesday’s primaries behind them, the candidates now look ahead to April 24, when Pennsylvania and four other states hold their primaries. Santorum, a former senator from Pennsylvania, can ill-afford to lose his home state and has keyed the future of his campaign to success there, a reality openly acknowledged by his advisers.

The urgency for Romney to pivot away from intraparty warfare toward the general election contest became even more apparent on Tuesday. On a day he formally clinched the Democratic nomination, Obama delivered a speech clearly designed to frame the fall choice. He sharply attacked the budget approved by House Republicans, calling it a “radical vision” and “thinly veiled Social Darwinism” that amounted to “a prescription for decline” in the country.

On Tuesday night, Romney fired back, calling the president out of touch with the suffering in society and saying that on Obama’s watch America isn’t working.

“This campaign is going to deal with many complicated issues but there is a basic choice we’re going to face,” he said in his victory speech in Milwaukee. “The president has pledged to transform America and he has spent the last four years building a government-centered society. I will spend the next four years rebuilding the foundation of an opportunity society led by free people and free enterprise.”

Already there is a rising chorus of party leaders urging that Republicans rally around Romney, while carefully avoiding direct calls for Santorum to get out. Party leaders fear that an extended contest that includes a continuation of the negative attacks that have been pervasive throughout the primaries will only weaken the party and its likely nominee for the fall.

Santorum runs the risk of either embarrassment, if he loses Pennsylvania, or becoming seen within his party as a spoiler if he stays in the race indefinitely and continues to attack Romney.

But on Tuesday night, he sounded a defiant note in his concession speech.

Saying the race was only at its halfway point, he vowed to keep campaigning and threatened to take his candidacy all the way to the GOP convention in Tampa. He laced his remarks, delivered in Pennsylvania, with criticisms of Romney, calling his rival a moderate who lacks true convictions.

“If we’re going to win this race we can’t have little differences between our nominee and President Obama,” he said. “We have to have clear contrasting colors.” He said people have gotten behind his candidacy “because they see someone who has a clear positive vision, someone whose convictions are also forged in steel, not on an Etch A Sketch.”

In the past two weeks, Romney has been endorsed by establishment leaders such as former Florida governor Jeb Bush and by two favorites of the right and the tea party, Florida Sen. Marco Rubio and House Budget Committee Chairman Paul Ryan (R-Wis.), the author of the House budget plan. Ryan was at Romney’s side throughout the final days of campaigning in Wisconsin.

Romney was likely to win the overwhelming share of the 95 delegates at stake on Tuesday. That would still leave him well short of the 1,144 delegates needed to secure the nomination, but the latest haul would expand what already was a significant lead over Santorum and the other two remaining candidates, former House speaker Newt Gingrich and Rep. Ron Paul (R-Tex.). Santorum was not even on the ballot in the District.

Tuesday’s contests were significant because they marked the first of two important days for Santorum’s candidacy-and what the Romney forces see as a critical three-week period that could effectively end a nomination battle that has lasted longer than many strategists had predicted when it began a few months ago.

Romney’s victory in Wisconsin was another major blow to Santorum. The former senator has had success in southern primaries but had lost Michigan, Ohio and Illinois to Romney in earlier contests and needed to demonstrate that he could expand his support beyond the narrow base that has backed his candidacy since he emerged as Romney’s main challenger.

Romney is favored to win all the other states except Pennsylvania with contests on April 24-New York, Connecticut, Delaware and Rhode Island. Santorum hopes that a victory in Pennsylvania will give him the legitimacy and the political lift to keep going into May, despite his underdog status. A poll released Tuesday showed Santorum leading in Pennsylvania by six points, but that margin is narrower than some earlier polls showed.

For many of the reasons the contests this month tend to favor Romney, the calendar in May is far more hospitable to Santorum. The May contests include primaries in North Carolina, West Virginia, Arkansas, Kentucky and Texas, states that have a significant number of evangelical Christians in the Republican electorate and that are similar in makeup to some of those Santorum already has won.

“If we win Pennsylvania, it sets up huge momentum going into the month of May, which we believe will be a great month for us,” said John Brabender, Santorum’s top strategist. “We think Pennsylvania will be do or die for both candidates.”

Santorum has said he will not consider dropping out of the race until it is clear that Romney has reached the delegate threshold to guarantee his nomination. The former governor began the day with almost 572 delegates, while Santorum was at 272, according to the Associated Press count.

Gingrich and Paul also have vowed to stay in the race for now, but they are not attacking Romney the way Santorum has been doing. Gingrich in particular has shifted his rhetorical focus from Romney to the president.