WATERLOO, Iowa – A Republican presidential campaign that has been slow to take shape suddenly snapped into focus Sunday, with an unlikely three-person top tier of former Massachusetts Gov. Mitt Romney, Rep. Michele Bachmann of Minnesota and the newest entry, Gov. Rick Perry of Texas.

After three eventful days — beginning with Thursday’s lively debate in Ames, Iowa, and running through Perry’s formal declaration of his candidacy, Bachmann’s victory in the Ames Straw Poll and former Minnesota Gov. Tim Pawlenty’s decision to drop out of the presidential race — the Republican Party is now looking at a nomination battle that is far different than the one envisioned at the beginning of the year.

Romney has long been considered the front-runner for the nomination, albeit one who has stirred only limited passion within the party.

But it is the emergence of Perry and Bachmann, who was lightly regarded until she began to demonstrate strong appeal to tea party and other conservative activists, that has dramatically changed the complexion of the race.

Bachmann was not considered a potentially strong candidate when the year began, but her victory in Saturday’s straw poll cemented her status in the upper ranks of the GOP field. Perry long claimed that he would never run in 2012, but the shifting circumstances of the Republican race through the first half of the year created a sizable opportunity that he has seized upon.

Bachmann’s victory also produced the first major casualty of the race, when Pawlenty, who finished third behind Bachmann and Rep. Ron Paul of Texas, abruptly ended his candidacy Sunday morning.

The Republican race is now a series of likely contrasts. Romney is cast as the establishment candidate who will portray himself as a former businessman who understands how to create jobs, and as the candidate who has the best chance of defeating President Obama in 2012.

Perry will challenge Romney on the economic front and will play on the anti-Washington message that he has been sharpening since Obama took office in early 2009.

Bachmann remains as the insurgent in the race. For those Republicans most upset with Obama, she will cast herself as the candidate who has fought him harder than anyone else.

In Perry, Romney faces a potentially formidable governor of a major state with an economic success story but who is mostly untested nationally.

The Texas governor, should he prove to be up to the challenges of handling a bruising presidential campaign, has the ability to stir passions among the party’s conservative base and reach into the GOP establishment for money and endorsements.

But Perry faces his own challenge in trying to fend off Bachmann, who begins this next phase of the campaign as the favorite of the tea party activists and who has impressed other Republicans with the energy she has brought to the race. Perry has an affinity with fiscal and social conservatives but will have to fight off Bachmann even as he hones in on Romney.

Pawlenty’s departure could change the landscape in Iowa. GOP strategists said Bachmann stands as the favorite to win the Iowa caucuses next winter. But Perry has said he will run and compete everywhere, and with Romney well-entrenched in New Hampshire, Iowa offers the Texan a chance to score an early victory and enhance his chances of winning the nomination.

Romney could reassess, though not necessarily change, his Iowa strategy, according to advisers. The former governor skipped Saturday’s straw poll and has shown little interest in competing to win the caucuses. But with Pawlenty out, and Bachmann and Perry competing for many of the same voters, a possible opening exists for Romney to make a stronger showing there.

The Romney campaign had anticipated that the straw poll might weaken Pawlenty, whom they considered a potentially formidable opponent. They did not expect him to be forced out so quickly. Now, Romney’s team is turning its attention to Perry, suggesting he will have to prove his mettle in Iowa and everywhere else, starting with a trio of debates next month.

How Perry performs in those debates will begin to answer the question of his readiness for a campaign that came upon him suddenly and for which he has had only minimal time to prepare.

But many Republicans say that, despite his late entry, they see him as a potential long-distance runner. As one put it Sunday, “Nobody thinks he’s another Fred Thompson.”

That was a reference to the disastrous campaign waged by the former Tennessee senator, who entered the 2008 race about this time four years ago and who never made a mark.

As Romney returns to the campaign trail today in New Hampshire, he plans to continue his assault on Obama and will highlight his private-sector experience and business know-how, drawing a subtle contrast with Perry.

The Texas governor has spent several decades in elected office, and in interviews Sunday, Romney strategists described Perry as a career politician.

Ray Sullivan, the Perry campaign’s communications director, said any criticism from rivals would be met with a sharp counterattack: “We will certainly and vigorously defend the governor’s record and vision.”