(AP Photo/Robert F. Bukaty)

PORTLAND, Maine - 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, a former Pennsylvania senator, said he could do "exceptionally well" in another industrial state: Michigan, where Romney's father served as governor. Michigan and Arizona host the next nominating contests in the state-by-state race 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.

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 per cent of the vote, narrowly defeating Texas Rep. Ron Paul with 36 per cent, state Republican chairman Charlie Webster said. Santorum and Newt Gingrich, the former speaker of the House of Representatives, won 18 per cent and 6 per cent, 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.

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

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 inMichigan, where Romney is a household name - and where his advisers had hoped for an easy victory. Romney'sfather, George, was chairman of now-defunct American Motor Corp. and was governor 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. In Maine, 61 per cent of voters selected a candidate other than Romney in state practically in his backyard. And Romney's showing was down considerably from 2008, when he won 51 per cent of the vote in Maine.

Former vice-presidential candidate 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."

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 U.S. Catholic bishops against the White House, Obama revised his policy. Instead of requiring church-affiliated non-profit 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.