MILWAUKEE -- The Obama administration launched a multipronged assault on Mitt Romney's values and foreign policy credentials yesterday, while a fresh set of prominent Republicans rallied behind the GOP front-runner as the odds-on nominee.

A defiant Rick Santorum outlined plans to leave Wisconsin the day before the state's contest Tuesday, an indication the conservative favorite may be in retreat, his chances to stop Romney dwindling.

"I think the chances are overwhelming that [Romney] will be our nominee," Senate Minority Leader Mitch McConnell said on CNN's "State of the Union." "And most of the members of the Senate Republican conference are either supporting him, or they have the view that I do, that it's time to turn our attention to the fall campaign and begin to make the case against the president of the United States."

Secretary of State Hillary Rodham Clinton and Vice President Joe Biden went after Romney yesterday, underscoring the belief at President Barack Obama's Chicago re-election headquarters that Romney will secure the right to face Obama.

"I think Gov. Romney's a little out of touch," Biden told CBS' "Face the Nation" in an interview broadcast yesterday. "I can't remember a presidential candidate in the recent past who seems not to understand, by what he says, what ordinary middle-class people are thinking about and are concerned about."

That line of attack is likely to play prominently in the Obama campaign's narrative. While Obama is a millionaire, Romney would be among the nation's wealthiest presidents elected.

Clinton seized on Romney's comment that Russia is America's "No. 1 geopolitical foe," suggesting there were more pressing concerns in global affairs.

But the administration's comments may have been overshadowed yesterday by Romney's ballooning Republican support.

Rep. Paul Ryan (R-Wis.) spent the weekend at Romney's side, campaigning across Wisconsin, one of three states to host Republican primaries Tuesday. First-term Sen. Ron Johnson (R-Wis.) followed Ryan's lead yesterday morning.

Ryan's endorsement was particularly painful for Santorum, who had been aggressively praising the congressman, a fiscal conservative hero in Wisconsin and across the country, for much of the past week. That praise ended Saturday, when Santorum referred to Ryan as "some other Wisconsinite."

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