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Fresh off a three-state defeat by a rival many had discounted, Mitt Romney is turning his line of attack to Rick Santorum, going after the former Pennsylvania senator on his record in Washington and for supporting earmarks. But many wonder whether Romney's new strategy can overcome the attacks on his social record.

Santorum's victories Tuesday underscore the point that conservative voters have yet to warm up to Romney. The former Massachusetts governor's business background is viewed favorably by Republican voters but many believe he's not conservative enough on social issues.

It also presents an additional challenge for Romney, who, in many ways, was campaigning as the candidate by focusing on President Obama and his administration. That strategy, experts say, is likely to change as the nomination extends into the summer.

"There is persistent unhappiness over and over again about Romney," political scholar Norm Ornstein said. "What he had hoped would happen after Florida… that he could put the other candidates in the rear view mirror and campaign as the nominee, he can't do that now."

Romney is going on the offense against Santorum, attacking the former senator's history on earmarks.  The former Massachusetts governor Wednesday said that during Santorum's time, "government grew by some 80 percent. He voted to raise the debt ceiling five times. This may work in Washington or it may have worked in the past, but it will not work in the future."

Santorum today fought back, charging that Romney has failed to present a clearer vision and defended his own record. He argued that many of the earmarks he supported were necessary for defense and health, such as the predator drone program and a human tissue medical program in Pittsburgh.

"There are good earmarks and bad earmarks… And when the abuse of the process became an issue, I stood up and said the process itself is not bad but the abuse is, and we need to stop it," Santorum, who is campaigning in Oklahoma City, said today. "He's interested in trying to pander and make political sauce, when there's real substantive issues about how we're trying to change this government, and he's on the wrong side."

Romney's new line of attack comes as there is increasing fervor against wasteful government spending and a push to trim spending. But it is unlikely to suppress concerns about his record that some conservatives believe is too moderate.

In an effort to change that, the former Massachusetts governor deployed two surrogates to tout his conservative record, Oklahoma State Treasurer Ken Miller and State Auditor Gary Jones. Both stressed the governor's record on fiscal issues and defended attacks by his rivals against his record as Massachusetts governor, including the state's controversial health care law.

"Gov. Romney is the most obvious in the race that can take the White House away from President Obama," Miller said in a conference call with reporters today.

Jones, who supported Mike Huckabee in 2008, defended Romney's time as governor of Massachusetts, adding that "it's a wonder he even became governor in Massachusetts," a liberal state.

"I think the main issue is the fact that getting the spending under control, managing the resources and the activities of the country no one is better equipped to do than Gov. Romney," he said.

Romney's campaign has sought to downplay the losses in Colorado, Minnesota and Missouri - called the "beauty contest" because no delegates will be awarded. They gave the example of Sen. John McCain, R-Ariz., in 2008, who lost 19 states but nabbed the nomination.

Romney also blamed the loss on Santorum's strategy of skipping the Nevada caucuses and focusing on the three states instead.

"Obviously, Sen. Santorum was able in Minnesota to level a lot of charges, some of them not accurate, and I wasn't there to respond," he told reporters Wednesday. "That was just because I happened to be fighting hard out in Nevada. But I will clearly make the differences between myself and my opponents very clear in where I think they have misrepresented my record and my views, and I'll make that very clear."

Romney remains the front-runner nationally, garnering 36 percent of the vote in Gallup's daily tracking poll. But the nomination fight is a long way from being over.

Santorum's triple-crown victory didn't necessarily help him win many delegates, but it does help him get headlines, money and more importantly, momentum, all of which Santorum struggled to gain in recent weeks as he was overshadowed by Romney and Gingrich.

Santorum, who also railed against Romney for "not talking about his record,"  raised nearly $1 million Tuesday and is set to have  another $1 million-day today, his campaign said.

The former senator from Pennsylvania now has the opportunity to leverage the skepticism conservatives have of Romney, like Gingrich, who used that to his favor in South Carolina. Santorum has focused his line of attack against Romney on social issues such as abortion and the Massachusetts health care law, which later became the basis of the national Affordable Care Act.

"I don't stand up here claiming to be the conservative alternative to Mitt Romney. I am here claiming to be the conservative alternative to Barack Obama," Santorum said Wednesday.

Gingrich is in Virginia today and will speak at Conservative Political Action Conference Friday. The former House speaker could face some challenge from Romney in his home state of Georgia, where he has a strong team and the money advantage.

Paul, meanwhile has his eyes set on Maine, where the state's Republican Party will announce its caucus results Saturday. The congressman from Texas has a strong network on the ground. He is the only Republican candidate who has visited the state during this campaign. But Romney, who is set to visit Maine Friday, won the state in 2008.

State GOP Chairman Charles Webster has said that "there are pockets of support for everybody" but "it's fair to say the race will come down to Romney and Paul."

ABC News' Shushannah Walshe, Emily Friedman and Jason Volack contributed to this report.