On the day that Texas Gov. Rick Perry formally joined the Republican presidential race, Rep. Michele Bachmann (R-Minn.) narrowly won the Iowa straw poll in a contest that dealt a major setback to third-place finisher Tim Pawlenty.

The events of Saturday marked the opening of an accelerating chapter in a 2012 GOP campaign that has been slow to take shape. With Mitt Romney established as the frontrunner for the nomination, the entry of Perry and the victory here by Bachmann are likely to reorder the field and intensify the competition to emerge as the former Massachusetts governor’s principal challenger.

Romney has dictated the pace of his campaign almost without regard to other contenders, and he has managed to avoid direct confrontations with the other Republicans in the field, most recently in Thursday’s debate. But that could change quickly as the GOP candidates look toward a September calendar that includes three nationally televised debates.

Bachmann captured 29 percent of almost 17,000 votes in the Ames straw poll and was closely followed by Texas Rep. Ron Paul, with 28 percent. Pawlenty received 14 percent. Former Pennsylvania senator Rick Santorum was fourth with 10 percent, and businessman Herman Cain was fifth, with 9 percent.

“This is the very first step toward taking the White House in 2012, and you have just sent a message that Barack Obama will be a one-term president,” Bachmann told supporters shortly after the results were announced.

Pawlenty’s disappointing finish threatens to end a candidacy that once held great promise. Until only a few months ago, many Republicans saw Pawlenty as a potentially strong candidate for the nomination. He has spent more time and money in Iowa than any other candidate.

In the run-up to the straw poll, which is not a reliable predictor of who wins later in the Iowa caucuses or finally receives the nomination, the former two-term Minnesota governor vowed to keep going no matter what the results were on Saturday. But he will have to reevaluate in light the tallies. By the time they were announced, Pawlenty and his senior team had packed up and left.

Pawlenty issued a statement congratulating Bachmann while claiming he had moved into a “competitive position” for next year’s caucuses. But he added, “We have a lot more work to do.”

Paul, who often runs strongly in straw polls, might have provided the biggest surprise of the day. But he has yet to demonstrate the broad appeal needed to win primaries or caucuses. Still, his finish is recognition that the iconoclastic conservative with the small-government message has greater resonance today than four years ago.

Perry formally announced his candidacy with a speech in South Carolina in which he criticized the policies of President Obama. “I will not sit back and accept the path that America is on because a great country requires a better direction, because a renewed nation needs a new president,” he said.

Perry has served longer than any other governor in Texas history and brings to the race solid conservative credentials, an affinity with both tea party activists and social conservatives, and an unbeaten record as a candidate.

But he is untested on the national stage and will have little margin for error, because he joins a race in which other candidates have been testing their messages and building organizations for months or even years.

Bachmann, too, is a relatively new candidate, having announced her candidacy only two months ago. But in that short time she has established herself as a politician with a passionate following and a message built around her claim that she has fought harder than almost anyone in the party to oppose the president’s policies.

But Perry threatens to eclipse Bachmann. For the time being, he is likely to take attention away from her candidacy and could drain some of the support she has developed. The two, who will challenge Romney from the right, have overlapping constituencies, though GOP strategists see Perry as having potentially broader appeal and a longer, deeper record, given his 10 years as a chief executive.

Both will visit Waterloo on Sunday, which could offer early clues.

Romney is seen as a less-than-dominant frontrunner; although he is ahead in most of the national polls, his candidacy has not generated much passion among many of the party’s conservatives. But he has one significant advantage over his main rivals: He has run before and knows what to expect.

Former Arkansas governor Mike Huckabee, who won the Iowa caucuses in 2008, was in Ames on Saturday and talked about what the race ahead might look like. On a day when many Republicans were asking what Perry would do to Romney, Huckabee said, “The question may be what Romney does to Perry. One thing Romney’s got going for him is that this is not his first rodeo. It’s a bruising experience.”

The straw poll turnout exceeded that of four years ago but was well below the 1999 record of more than 23,000. Perry was not on the ballot but, aided by an energetic write-in operation, finished sixth, just ahead of Romney, who was on the ballot. Former House speaker Newt Gingrich was eighth, followed by former Utah governor Jon Huntsman Jr. and Rep. Thaddeus McCotter of Michigan.

A victory by Paul would have been a blow to Bachmann, who brought considerable momentum to the vote and needed the victory to validate her standing as the frontrunner in the state. It also would have hurt the credibility and future of the straw poll, a number of Republicans here said.

Saturday’s straw poll drew a bigger-than-expected crowd, which reveled in the early fall weather, with blue skies, moderate temperatures and a slight breeze. By midday, traffic heading onto the Iowa State University campus was moving at a crawl.Once in the area, the candidates’ supporters faced long lines to vote.

As the voting took place, each of the candidates, flanked by their families and small armies of supporters, filed onto the floor in the Hilton Coliseum. Some of the campaigns played quick videos on the JumboTron above before their candidates took the microphone.

Few generated more enthusiasm than Bachmann, who confidently roamed the stage with a wireless microphone rather than standing still at the rostrum.

“We are going to do it. It’s going to happen — 2012 is ours,” Bachmann said as the nearly full coliseum erupted in cheers. “We’re going to take it back in 2012. All the energy we need to take the country back in 2012 — it’s right here in this room, in Ames, Iowa.”

Bachmann offered few details, delivering instead a sharp critique of Obama and an expression of her conservative principles, economic and social. She drew perhaps the loudest applause when she congratulated Iowans for voting out three state Supreme Court justices last year who had ruled against a law barring same-sex marriage.

Pawlenty, too, offered a withering critique of Obama’s Washington. He promised to reduce taxes, spending and regulation — and, as he has done everywhere on the trail over six weeks of intense campaigning across Iowa, he made the case that his record as governor distinguishes him from the similar promises of the rest of the field.

“Many of the Republican candidates for president are going to say many of the same things,” he said. “They’re going to say, ‘I’m the one who will reduce taxes. I’m the one who stands for the cause of life, I’m the one who stands for traditional marriage.’ But as you know, as we’ve seen with Barack Obama, just saying the words isn’t enough.”

As he has throughout his campaigns for president, Paul set himself apart from Republican orthodoxy. Like Bachmann, he drew a strong response. His ardent supporters, who filled much of the arena, cheered loudly as he made a passionate call for a return to the principles laid out in the Constitution, reforming monetary policy at the Federal Reserve and bringing U.S. forces home from throughout the world.

“We’re into wars that are costing us trillions of dollars,” Paul said. “Those wars are costing us jobs and prosperity.”

Santorum, who moved his family to the state last month and who visited 68 of the state’s 99 counties, called his campaign “the little engine that could,” saying, “This campaign is about scratching and clawing for every bit of recognition we can get.”He implored Iowa Republicans to recognize his conservative convictions as more deeply held than those of some of the others in the race.

Cain gave perhaps the most rousing speech of the afternoon, laying out his solutions for the country’s many “crises” and suggesting he is the product of a movement of citizens ready to take back their government. “This giant has awakened, and it’s not going back to sleep,” he said.

Displays of enthusiasm were even more pronounced on the lawns surrounding the Hilton Coliseum, where Bachmann supporters waited in a two-hour line to gain admission to her tent, eat a “beef sundae” and listen to country star Randy Travis perform.

In Pawlenty’s tent on the other side of the coliseum, organizers dished out 4,000 Dairy Queen Blizzards and 1,750 pounds of Famous Dave’s BBQ, but by midafternoon the crowds had dwindled and more tables sat empty than full inside a cavernous tent.

Staff writers Aaron Blake, Chris Cillizza, Philip Rucker and Karen Tumulty contributed to this report.