(AP Photo/Robert F. Bukaty)

WASHINGTON - The Republican presidential candidates faced a more than two-week break in the primary and caucus calendar Monday, a breather 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. Former Massachusetts governor Mitt Romney, the favourite of the party establishment, is well out in front in the total number of delegates to the party's national convention in August. But a surprising challenge from former Pennsylvania Sen. Rick Santorum has Romney scrambling to find a message that resonates with the deeply conservative Republican base that refuses to warm to his candidacy.

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 quiet time to raise much-needed cash in California. He insisted he's in the race for the long haul, dismissing calls from Santorum and others to drop out. He said he's been counted out in the presidential campaign several times before but that his candidacy has rallied back each time.

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.

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.

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.

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.

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 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.