Mitt Romney unveiled plans Thursday for an aggressive finishing sprint in Iowa designed to lock down a victory in Tuesday’s caucuses that would leave his rivals scrambling to catch up.

Romney is far from a clear favorite in Iowa: Rep. Ron Paul (Tex.) continues to show strength in the polls and is banking on a well-regarded organization, and former senator Rick Santorum (Pa.) is on the rise. But no campaign can match Romney’s for the breadth and depth of its infrastructure, and for the first time the weapons he can deploy are all on display.

For months, Romney’s campaign in Iowa appeared to be moving in slow motion, but it has suddenly taken on a new intensity and some Republican ­strategists say the former Massachusetts governor is building his Iowa momentum at just the right time.

The volatile political landscape in Iowa underwent its latest jolts in the past 36 hours, scrambling the fortunes of several candidates — most notably Santorum, Paul and Newt Gingrich — and left Romney in his most favorable position in Iowa so far.

In a year that has produced one candidate surge after another in Iowa, Santorum on Thursday was trying to capitalize on a new poll by CNN and Time magazine showing that he has tripled his support in December and is now running third behind Romney and Paul.

Claiming that the Obama White House fears his candidacy more than that of any other Republican, the former senator kept up his steady campaign pace, but in the predictable rhythm of today’s politics, Santorum’s rise made him the latest target of scrutiny and attacks from some of his rivals.

Santorum’s move came in part as a result of a steep slide by Gingrich, who has been barraged by negative TV ads for the past several weeks. A new analysis by Kantar Media’s Campaign Media Analysis Group reported Thursday that 45 percent of all the advertising in Iowa since Dec. 1 has been negative ads aimed at Gingrich. Only 20 percent has been negative ads aimed at Romney.

The other candidates — Texas Gov. Rick Perry and Rep. Michele Bachmann (Minn.) — pressed forward against the odds, hoping that Iowa voters, who have been changing their minds for months, will deliver one last unexpected twist to the narrative.

But increasingly, the focus of the race is Romney and his potential to win Tuesday. He is drawing energy from boisterous crowds, some of whose participants said in interviews that they had only recently come around to Romney because he seems like the party’s best bet to beat President Obama.

Romney asked Iowa voters Thursday to weigh their choices carefully.

“I hope as you look at the people running, you can measure their capacity to lead effectively and you can also determine whether they can become our nominee and defeat President Obama,” Romney told 400 supporters in Mason City. “I think I can.”

Romney has scheduled a statewide dash that seems to answer affirmatively the question of whether he believes he can win. He will finish his three-day bus tour at a rally Friday morning with New Jersey Gov. Chris Christie. Then, after a 30-hour jaunt to New Hampshire, where he is working to convert a commanding lead in the polls into victory on the ground, Romney will return to Iowa with a string of rallies in the bigger cities in advance of Tuesday’s voting.

Romney has worked for months to play down expectations in the state that dealt a severe blow to his candidacy four years ago. But there was no mistaking the growing confidence inside the campaign just five days before precinct caucuses here kick off the Republican nomination process. Romney arrived at his final event of the day, a rally in Ames, by driving his campaign bus behind the stage as “Eye of the Tiger” played loudly.

A Romney victory in Iowa, coupled with an expected win in New Hampshire, where he has maintained a wide lead over all his rivals, would put him in a commanding position heading into the Jan. 21 South Carolina primary — a contest that in past Republican races proved decisive — and the Jan. 31 primary in Florida.

Polls show that a significant percentage of Iowa Republicans may yet change their minds before Tuesday, which has increased the pressure on all the candidates. In comparison to past campaigns here, the absence of a clear hierarchy in the field has made it more difficult to set and stick to a strategy.

Paul had received little direct criticism until recently, but most of the attacks have been in statements by the candidates, rather than from television ads, which remain the most effective way to deliver a negative message.

Santorum is now under fire, but there will be little time for his opponents to make their criticisms of him stick before Tuesday. And Romney, although he long has been seen as at least a nominal front-runner in the race, has escaped the kind of attacks normally leveled at someone in his position.

Gingrich has remained upbeat in the face of his slide in the polls, but he acknowledged in an interview that it’s too soon to say how long the damage from the attacks may last. Whether he can rebound after the onslaught is the decisive question facing his candidacy. He has vowed that he will be ready for battle in South Carolina.

Gingrich said he thinks that his message — that he is the rightful heir to the Reagan supply-side conservatism of the 1980s — will play well there in contrast to Romney’s record as governor in Massachusetts.

Still, a weak finish on Tuesday could be crippling.