Wisconsin exit polls show that endorsements might have helped Romney. About 60% of respondents said the Romney endorsements were a factor in their votes, and 33% called them an "important" factor. Most of those voters broke for Romney.
Santorum's campaign points toward May, which looks friendlier for him. He could be the favorite in primaries in North Carolina and West Virginia on May 8, Nebraska on May 15, Arkansas and Kentucky on May 22 and Texas a week later.
But it could be too little, too late by May. Sen. Hillary Clinton won most of the final contests against Sen. Barack Obama in the marathon 2008 Democratic presidential nomination battle. But in the end, it didn't matter.
And Ari Fleischer, a CNN contributor and former George W. Bush White House spokesman, said that while he believed that Santorum earned the right to fight through Pennsylvania, the race was all but over.
"I think the writing is on the wall. Does Rick Satorum want to read it is the question," Fleischer said.
Romney and Obama on Tuesday seemed to be turning their attention to November's general election, with Romney blasting the president during a campaign stop in Wisconsin and Obama mentioning Romney in a speech for the first time this year.
Speaking at a restaurant in Waukesha, Wisconsin, Romney suggested Obama wants to duck responsibility "for what's happened in this country," saying the president should get full credit or blame for "what's happened in this economy, and what's happened to gasoline prices under his watch."
"It is time to have somebody who will take responsibility, and if I am president, I will not only get things right again, I will take full responsibility for my errors and make sure that people understand we have a president in the White House again where the buck will stop at his desk," Romney said, while standing with House Budget Committee Chairman Paul Ryan, a prominent Wisconsinite who endorsed Romney.
Later Tuesday, Obama mentioned Romney while slamming a House-passed budget proposal that Ryan drew up and Romney embraced.
Obama, speaking at a media luncheon in Washington, said the plan, which would lower tax rates and cut spending while reforming the Medicare and Medicaid government-run health care programs, was "thinly veiled Social Darwinism" and "antithetical to our entire history as a land of opportunity and upward mobility for everyone who's willing to work for it."
"One of my political opponents, Gov. Romney, has said that he hoped a similar version of this plan from last year would be introduced on day one of his presidency," Obama said. "He said that he's very supportive of this new budget and he even called it marvelous, which is a word you don't often hear when it comes to describing a budget."
While Romney is far ahead of Santorum, Gingrich and Paul in the hunt for delegates, exit polls have indicated Romney still has a problem locking in the conservative base of the party.
That continues to fuel Santorum's campaign, in which he continually depicts himself as the lone true conservative going up against the Republican establishment and liberal media bias.
As in previous contests, Wisconsin's early exit polls showed Romney doing better among higher-earners and Santorum better with lower-income voters. Among exit-poll takers making $100,000 to $200,000 annually, Romney led 55%-30%, with Santorum winning the under-$30,000 voters, 44%-34%.
The Wisconsin exit polls showed Santorum is more popular in rural areas and Romney in urban areas in Wisconsin. According to the exit polls, Santorum was winning the rural vote, 37%-27%, and Romney was winning big-city voters, 43%-23%.
About 24% of the vote in Wisconsin came from urban areas and 22% from rural areas, according to the exit polls. The rest is suburban.
Wisconsin's primary was open to any registered voter in the state, regardless of party. Exit polls showed that 58% described themselves as Republicans, 30% said they were independent and 12% said they were Democrats.
Santorum was doing best among self-described Democrats with 32% of their vote, while Romney had 22% and Paul had 16%.