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Mr. Romney, buoyed by growing signs that Republicans are coalescing around his candidacy, has in recent days focused his rhetoric on Mr. Obama.

He’s also tasked Beth Myers, a longtime adviser, with leading his search for a potential running mate, even stumping with some of the potential candidates, including Sen. Marco Rubio of Florida, who campaigned with the front-runner this week in the Philadelphia suburbs.

With a virtually insurmountable lead in convention delegates over the two remaining Republican contenders, Mr. Romney has circled back to the economy and the “Barack Obama has failed America” message that he opened his campaign with last summer.

He also has showed signs of tacking to the middle on some issues, basically endorsing Mr. Obama’s call to reduce student-loan interest rates for college students and keeping the door open to the possibility of supporting a Dream Act proposal being drafted by Mr. Rubio that would create a pathway to legal status, but not citizenship, for some children of illegal immigrants.

Mr. Romney’s emergence as the presumptive winner of what was once a crowded Republican field became clear earlier this month when he easily defeated former Sen. Rick Santorum of Pennsylvania in the Wisconsin primary — part of a three-contest sweep that included the District of Columbia and Maryland.

Mr. Santorum, a steadfast social conservative, initially vowed to fight on at least until Pennsylvania. But days later, he suspended his campaign after it became clear that his fundraising was drying up and that he could lose his old political backyard again, having lost the state by 18 points in his 2006 Senate re-election bid.

Mr. Santorum’s exit paved the way for Mr. Romney to sew up the nomination before the Republican National Convention this summer in Tampa, Fla.