With two new polls showing underdog Rick Santorum gaining on front-runner Mitt Romney among Republicans nationally, the contenders for the GOP presidential nomination are taking advantage of a lull between primary contests to stockpile resources and rally supporters for the next phase of the race.

Separate surveys released Monday by Gallup and the Pew Research Center suggest that Romney, a former Massachusetts governor, and Santorum, a former senator from Pennsylvania, are virtually tied.

Santorum’s apparent surge comes after a string of victories last week in Missouri, Minnesota and Colorado, and it reflects the dissatisfaction and unease that grass-roots conservatives continue to feel about Romney, who has the support of most of the GOP establishment and far superior financial and organizational resources.

The former senator appears to have unseated former House speaker Newt Gingrich (Ga.) as the conservative alternative to Romney in this very fluid race, which has seen more twists and turns than any GOP nominating contest in memory.

The dynamic has become familiar: A challenger to Romney’s dominance emerges, only to fall under an onslaught of scrutiny and negative advertising.

But the fact that Romney remains unable to escape the political tar pit in which he finds himself is worrisome to many Republicans who believe he is the most electable candidate against President Obama.

Two weeks lie between now and the next contests, in Michigan and Arizona, making this the longest break since the primary season officially got underway with the Iowa caucuses in early January.

Romney plans to spend much of this week continuing to raise money for what looks to be a longer and more expensive nominating battle than some on his team initially expected.

As his aides prepare an advertising campaign across the 10 states that will vote on “Super Tuesday,” March 6, his fundraisers are coming off one of the most successful weeks of his campaign. Romney hosted major events last week in Denver, Atlanta and Washington, the last of which brought in between $1.3 million and $1.5 million, according to a top Romney fundraiser.

Romney’s aides also are urging his conservative supporters to take a more public role in the campaign. One adviser said his conservative and tea-party-aligned backers in Congress and in the GOP’s grass roots will be more outspoken in their support for Romney, through television and radio appearances and media interviews in targeted markets.

The campaign also may try to mobilize the many conservative talk radio hosts who are backing Romney to help dispel the notion that he has weak backing among conservatives.

At a speech Monday at an outdoor amphitheater in Mesa before about 2,500 people, Romney did not mention any of his Republican opponents directly, instead keeping his fire trained on Obama.

“This is really a battle for the soul of America,” he said. “It’s essential that we win this election and we get Barack Obama out of the White House.”

Santorum spent Monday in Washington state, kicking off a western swing on the same day the state became the seventh, along with the District, to allow same-sex marriage, an issue he has frequently brought up on the campaign trail.

A favorite of evangelical conservatives, Santorum has seen large, enthusiastic crowds; in Oklahoma last week he spoke to 4,000 people at Oral Roberts University.

His team continues to focus on smaller contests, a strategy that helped him win last week. Yet aides said he expects to gain traction in Michigan and Ohio, where his plan to zero out the federal tax rate for manufacturers is likely to find an audience among blue-collar workers hit hard by downsizing in the manufacturing sector.

Romney, however, has a home-court advantage in Michigan, given the fact that his father was governor of the state and that he was raised there. He won the state’s primary four years ago.

Gingrich, meanwhile, embarked on a four-day tour of California on Monday focused primarily on raising money. His first public event, at the Cielito Lindo restaurant in South El Monte, drew only a few dozen supporters.

He was scheduled to make a handful of public appearances in the Los Angeles area and in Fresno, but he planned to appear at multiple fundraisers each day in those cities as well as in San Diego and San Jose.

Gingrich is launching yet another new strategy as his campaign tries to recapture momentum after Santorum’s wins last week. He told reporters that his previous plan to try to establish a two-man race with Romney won’t work now. Aides said they see the race turning into a Gingrich-Santorum face-off.

“Our strategy has to change,” Gingrich said. “A strategy of having better, positive, bold ideas. Read the speech I gave at CPAC,” he said, referring to the Conservative Political Action Conference last week.

“I think my ideas are much bolder than Santorum or Romney’s,” he continued. “I think my ideas are much clearer and more specific, and I have to focus on communicating those ideas. The two periods I focused on communicating those ideas I ended up No. 1 in Gallup both times. And we’re going back to doing what we did that worked.”

Rep. Ron Paul (Tex.), the fourth remaining contender in the GOP field, has not announced any public events between now and a Feb. 22 debate.

Staff writers Philip Rucker in Washington and Nia-Malika Henderson in Tacoma, Wash., contributed to this report. Gardner reported from California.