PORTLAND, Maine (AP) ― A day after front-runner Mitt Romney regained some momentum in the Republican presidential contest, he faced the task of convincing Republican voters he is conservative enough to be the party standard-bearer and fending off a spirited challenge from rival Rick Santorum.

Santorum went on the attack Sunday, calling the former Massachusetts governor and wealthy businessman “desperate” while promising to compete aggressively to win the upcoming primary in the state where Romney grew up. Santorum said 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.

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.

Romney has been painting Santorum as a longtime 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 Republican 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.

Santorum wasn’t a factor in Maine, where Romney captured 39 percent of the vote, narrowly defeating Texas Rep. Ron Paul with 36 percent, state Republican chairman Charlie Webster said. Santorum and Newt Gingrich, the former speaker of the House of Representatives, won 18 percent and 6 percent, respectively.

Santorum shrugged off his third-place finish Saturday in Maine, where he didn’t actively compete, as well as his second-place finish to Romney in a straw poll of conservative activists meeting in Washington. Instead, Santorum said he was looking ahead to the next round of primaries.

“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 television’s “This Week.’’ He suggested that a strong showing in those contests would make the presidential contest “a two-man race,’’ dismissing current rivals Gingrich and Paul.

As Santorum eyes Michigan and Arizona, Romney was turning his attention to extending his huge cash advantage over his rivals.

Romney 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. “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. His team is preparing an aggressive push against Santorum ineuroMichigan, where Romney is a household name _ and where his advisers had hoped for an easy victory. Romney’seurofather, George, not only served as governor in the state but was chairman of now-defunct American Motor Corp. before mounting a failed bid for president in 1968.

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 northeastern state practically in his backyard. And Romney’s showing was down considerably from 2008, when he won 51 percent of the vote in Maine.

Former Alaska Gov. Sarah Palin, a prominent voice among some conservatives, said Romney has work to do to convince Republican 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 (Republican) and independent voters are convinced,’’ Palin, the party’s 2008 vice presidential nominee, said on “Fox News Sunday.”

With Americans concerned about high unemployment, Romney has emphasized his business background to try to convince voters that he’s best prepared to turn the economy around. Obama has been considered vulnerable in his bid for a second term in the White House because of his handling of the economic downturn spawned by the near collapse of the U.S. financial system in the final months of the presidency of George W. Bush.

That disadvantage with voters seems, however, to be easing with increasing but tentative signs of a slight acceleration of the economic rebound. As a result, Republican candidates have started emphasizing social issues more, which plays to Santorum’s strength as staunch cultural conservative opposed to abortion and gay marriage.

To counter his challenge, Romney has also pivoted to social issues in recent days. Romney has been particularly aggressive in criticizing Obama’s recent decision regarding contraception which has become a hot-button issue for conservatives.

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 said 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.”

Coming off last week’s success, Santorum saw a surge in donations. His campaign reports gathering $3 million in the three days immediately following last Tuesday’s sweep, but he’s unlikely to catch Romney who has a huge advantage in the money race and organizational strength.

The Maine results were disappointing for the libertarian-leaning Paul, who is still seeking his first victory in a state nominating contest after a handful of top three finishes in other early-voting states.

Romney won 11 delegates and Paul 10, according to an analysis of the Maine results by The Associated Press. That brings the delegate count to 123 for Romney, 72 for Santorum, 32 for Gingrich and 19 for Paul, with 1,144 delegates needed to capture the nomination at the Republican National Convention in late August in Tampa, Florida.