Santorum said Sunday he could do "exceptionally well" in Michigan, where Romney's father served as governor. The Midwestern state and Arizona host Republican presidential nominating contests on Feb. 28.
"We're going to spend a lot of time in Michigan and Arizona, and those are up next. And that's where we've really been focusing on," Santorum told ABC's "This Week." He suggested that a strong showing in those contests would make the presidential contest "a two-man race," dismissing current rivals Newt Gingrich and Ron Paul.
Santorum shrugged off his third-place finish Saturday in caucuses in Maine, where he didn't actively compete, as well as his second-place finish in a straw poll of conservative activists.
Romney has been painting Santorum as a long-time Washington insider who pursued home-state projects. Santorum on Sunday described Romney's recent criticism as "desperate."
"You reach a point where desperate people do desperate things," said Santorum, who represented Pennsylvania during his 16 years in Congress, first in the House and then in the Senate.
Maine GOP officials declared Romney the winner of Saturday's caucuses. The results ended a three-state losing streak to Santorum, who swept contests in Colorado, Minnesota and Missouri on Tuesday.
With the next primaries more than two weeks away, the break seems unusually long in the rapid-fire race that's featured six contests in the last 14 days. Romney and his rivals now have 17 days to raise cash and bolster their organizations for what's shaping up to be a slog to the Republican nomination and the right to face President Barack Obama in November.
As Santorum eyes Michigan, Romney turns his attention to extending his huge cash advantage over his rivals.
The Massachusetts governor left Maine before the caucus results were announced to attend a West Coast fundraiser Saturday night. He issued a written statement to mark his victory in the low-turnout contest.
"I'm heartened to have the support of so many good people in this great state," Romney said in the statement. "The voters of Maine have sent a clear message that it is past time to send an outsider to the White House."
Romney is expected to spend much of next week courting donors, while sprinkling in a handful of campaign events. He'll be in Arizona Monday evening.
Romney won a plurality of the Maine vote just hours after winning the presidential straw poll at the Conservative Political Action Committee conference in Washington.
But questions about Romney's durability as his party's presumed front-runner persist. Fully 61 percent of Maine voters selected a candidate other than Massachusetts' former governor in a state practically in his backyard. And Romney's showing was down considerably from 2008, when he won 51 percent of the vote.
Former Alaska Gov. Sarah Palin, a prominent voice among some conservatives, said Romney has work to do to convince GOP voters he's moved beyond his "pretty moderate past ... even in some cases a liberal past."
"I am not convinced, and I do not think the majority of GOP and independent voters are convinced," Palin said on Fox News Sunday.
Romney has focused more on social issues in recent days. He has been particularly aggressive in criticizing Obama's recent decision regarding contraception.
On Friday, after three weeks of controversy that pitted the nation's Catholic bishops against the White House, Obama revised his policy. Instead of requiring church-affiliated nonprofit employers to cover free contraception with the health insurance they offer workers, the policy now requires insurance companies to provide free birth control coverage in separate agreements with workers who want it.
White House chief of staff Jack Lew defended the decision Sunday, noting that there is no longer room for compromise.
"This is our plan," he said on CNN's "State of the Union."
Santorum said the president's plan doesn't resolve the issue. He says many Catholic institutions are self-insured and those organizations will still be forced to pay for women's contraception despite their religious objections.
"There's no compromise here. They are forcing religious organizations, either directly or indirectly to pay for something that they find is a deeply, morally, you know, wrong thing," Santorum, a Catholic, told NBC's "Meet the Press."
The focus on social issues plays well for Santorum, who has long been considered a staunch cultural conservative. Those credentials helped fuel his success last week.
But he wasn't a factor in Maine, where Romney captured 39 percent of the vote, narrowly defeating Paul's 36 percent, state Republican chairman Charlie Webster said. Santorum and Gingrich, who didn't actively campaign in Maine, won 18 percent and 6 percent respectively.
It was a disappointing showing for Paul, who on Sunday suggested that there was virtually no difference between his rivals.
"All three of them have represented the same system, the same status quo," Paul said on CBS' "Face the Nation."
The Maine vote totals reflected about 84 percent of the state's precincts, with nearly 5,600 Republicans voting out of 258,000 registered. The contests scheduled for the coming days will not be counted, including an entire county that postponed its caucuses because of a snow storm.
"We were a little bit disappointed last night," Paul said, because he's done well in that county in the past and expected to do well Saturday.
Coming off last week's success, Santorum saw a surge in donations. His campaign reports gathering $3 million in the three days immediately following after last week's hat trick, but he's unlikely to catch Romney in the money race.
Santorum reported just $279,000 in the bank at the end of December, compared with Romney's $19.9 million. Gingrich had $2.1 million, but is still carrying substantial debt, while Paul reported $1.9 million.
Romney won 11 delegates and Paul 10, according to an analysis of the Maine results by The Associated Press. Santorum and Gingrich were shut out. That brings the delegate count to 123 for Romney, 72 for Santorum, 32 for Gingrich and 19 for Paul, with 1,144 delegates needed for the nomination.
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