(AP Photo/Charlie Riedel, File)

WASHINGTON - Front-runner Mitt Romney was scrambling to fend off a surprising challenge from Rick Santorum as the Republican presidential candidates faced a more than two-week break before the next primary contests in the frantic contest for the party's nomination to challenge President Barack Obama in November.

The race has included primary elections and caucuses in 10 states since the year began. Romney, the former Massachusetts governor who is the favourite of the party establishment, is well out in front in the total number of delegates to the party's national convention in late August. But Santorum, a former Pennsylvania senator, has left Romney struggling to find a message that resonates with a deeply conservative Republican base that refuses to warm to his candidacy.

A new poll released Monday by the Pew Research Center for the People & the Press showed Santorum surging strongly among tea party and white evangelical voters in the Republican party. The Pew survey of Republican and Republican-leaning voters showed Santorum with 30 per cent support to 28 per cent for Romney, a statistical tie. That compares with a Romney advantage of 31 per cent to 14 per cent a month ago.

Santorum's new strength was especially notable among the most conservative Republicans - the tea party and white evangelicals -where the former Pennsylvania senator was streaking ahead with a 41-23 percentage-point margin.

Former speaker of the House of Representatives Newt Gingrich, who looked to be stealing the front-runner position after winning the South Carolina primary, will be using this lull in primary and caucus contests to raise much-needed cash in California.

Gingrich insisted he's in the race for the long haul, dismissing calls from some prominent conservatives for him to drop out of the presidential contest in order to set up a direct contest between Romney and Santorum. He insisted that his ideas and a new determination to stay positive would help him once again resuscitate his flagging candidacy.

"I think my ideas are much bolder than Santorum or Romney's. I think my ideas are much clearer and more specific and I have to focus on communicating those ideas. Let's see how it plays out," Gingrich told reporters after addressing a Hispanic leadership event near Los Angeles.

Speaking to a tea party group in Pasadena, Gingrich depicted Santorum as a flash in the pan whose own candidacy had been nearly left for dead before he won contests in Minnesota, Missouri and Colorado last week.

"He had a really good Tuesday and suddenly the same people who said I was dead in June are saying, 'See, I told you so,'" Gingrich said. "I have a message for them - I'm still here."

Ron Paul, the libertarian-leaning Texas congressman with a small but devoted following and who nearly beat Romney in the latest caucuses in Maine on Saturday, plans to continue campaigning through what promises to be a long and drawn-out race for the nomination.

Romney captured 39 per cent of the Maine vote, just three percentage points over Paul. Santorum and Gingrich, neither of whom campaigned hard in the state, took 18 per cent and 6 per cent of the vote, respectively.

At an outdoor rally with about 2,500 people in Mesa, Arizona, Romney didn't mention Santorum or Gingrich by name, but he tried to paint both as longtime Washington insidersr.

Romney told the Mesa crowd that Paul was a doctor before entering Congress. But he said "the other guys have spent their life entirely in government."

He was referring to Gingrich and Santorum, both of whom spent about two decades in Congress.

"Let's not nominate somebody who has not run anything and has not been a leader," Romney said.

Santorum has waved off Romney's recent criticism. "You reach a point where desperate people do desperate things," he said Sunday.

The four remaining Republican candidates, including Ron Paul, will meet for a nationally televised debate Feb. 22 in Arizona, which holds a primary Feb. 28, as does Michigan.

Santorum said he was looking ahead to the next round of primaries after Maine and his second-place finish to Romney in a straw poll of conservative activists meeting in Washington last week.

Santorum had his campaign fixed on the next primary contests in Michigan and Arizona. Romney is expected to spend much of the week courting donors.

Romney also planned to campaign Wednesday and Thursday in Michigan, where he was born. Romney'sfather, George, was chairman of the now-defunct American Motor Corp. and was governor of the state before mounting a failed bid for president in 1968. But despite Romney's advantages, a poll of Michigan Republican voters released Monday by Public Policy Polling showed Santorum at 39 per cent to 24 per cent for Romney, with Gingrich bleeding support at 11 per cent.

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 a state that neighbours the one where he was governor. And Romney's showing was down considerably from 2008, when he won 51 per cent of the vote in Maine.

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 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.

With tentative signs of a slight acceleration of the economic rebound, Republican candidates have started emphasizing social issues more. That plays to Santorum's strength as a staunch cultural conservative opposed to abortion and gay marriage.

The four remaining Republican candidates, will meet for a nationally televised debate Feb. 22 in Arizona, which holds a primary Feb. 28, as does Michigan.