MESA, Ariz. – With 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.

New polls this week suggest Romney, former Massachusetts governor, and Santorum, 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 as the conservative alternative to Romney in this fluid race.

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.

As Romney’s 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.

Santorum, who has cast himself as the only true conservative in the race who can compete with Obama on issues important to Republicans, spent Tuesday in Idaho, which will have caucuses March 6.

Gingrich, meanwhile, embarked on a four-day tour of California on Monday focused primarily on raising money.