Romney has 572 Republican National Convention delegates, exactly half of the 1,144 needed to win the nomination, and is on a pace to clinch by the end of the primary season in June.

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Santorum has 272 delegates, Newt Gingrich 135 and Ron Paul 51.

There were 95 delegates at stake Tuesday, including 42 in Wisconsin, the only one of the three contests that Santorum seriously contested.

Romney is expected to do well in Maryland and in the District of Columbia, where Santorum's name doesn't appear on the ballot.

The former Pennsylvania senator has retreated to his home state to watch returns from Wisconsin, holding an evening event in Mars, a town outside Pittsburgh.

Romney has ignored Santorum the past few days and focused instead on Obama, telling supporters in Green Bay that the president "takes his political inspiration from the capitals of Europe."

Obama's ad claims that "Mitt Romney's stood with Big Oil - for their tax breaks, attacking higher mileage standards and renewables."

It's a response to an ad from Crossroads, a Republican super PAC that's expected to spend millions of dollars attacking the president this year.

Romney's campaign, though, is running far behind the president in fundraising, as he's been unable to raise general election money because of the drawn-out primary contest.

At the end of February, Obama reported $84.7 million in his campaign account compared with Romney's $7.3 million.

Obama has more than 530 paid staff compared with roughly 100 for Romney. But Romney has far outspent his rivals during his bid for the nomination.

Santorum, also campaigning in Wisconsin on Monday, said Romney has essentially bought his success by outspending the competition.

Romney and his allies have spent a combined $53 million on television advertising so far this election cycle compared with just $27 million from his three Republican competitors combined, according to data compiled by the media tracking firm SMG Delta.

Santorum's team, having narrowly lost a string of high-profile contests, spent just $9 million.

"With almost unlimited resources, Gov. Romney has not proven to be very effective," Santorum said as he predicted a possible upset in Wisconsin.

"The only way he's been successful in winning the primaries is by just bludgeoning his opponents by an overwhelming money advantage - something he's not going to have in the general election."