No dramatic gaffe or sudden change in direction explains how Rep. Michele Bachmann of Minnesota fell out of favor with Republican voters — almost as quickly as she won them over when she joined the presidential race in June.

What does explain Bachmann’s precipitous rise and fall is the fact that Republicans, despite their urgent desire to defeat President Obama next year, still aren’t sure exactly what they want in his replacement.

“Voters want someone who is right on the issues, they want a fighter, and they want someone who can beat Obama,” said Republican pollster Jon Lerner. “When you look at the candidates who have had a lot of motion in their poll numbers, first Donald Trump, then Herman Cain, then Michele Bachmann, what do they all have in common? They were all perceived as conservatives and fighters, which led to their rise, but there was uncertainty about whether they were winners, which contributed to their fall.”

Buffeted by those competing impulses, voters have spent much of the year toying with one candidate after the next, only to decide they’re still looking. This uncertainty has been both a boon and a bane to different candidacies and different times, but it is especially tricky for the front-runners.

Now it is Texas Gov. Rick Perry’s turn.

Perry is a crowd-pleasing campaigner whose conservatism, combined with 10 years of executive experience, helped him quickly eclipse Bachmann and the rest of the field in polls soon after he entered the race last month. Yet he remains unproven for a big swath of the electorate. With his fiery rhetoric and unapologetic critiques of Social Security and the federal income tax, Perry has had no trouble rallying the GOP’s conservative wing. But he has yet to convince a broader audience that he is ready to face off against Obama.

Whether Perry can avoid that Bachmann’s boom-to-bust fate will be shaped largely by how he defines himself at a series of presidential debates scheduled in the coming weeks, including one Monday evening in Tampa. A crucial factor is whether former Massachusetts governor Mitt Romney, the onetime front-runner and Perry’s main rival for the nomination, defines Perry first. Increasingly, the nomination fight is being framed as a contest between the two men.

Like Bachmann’s rise and fall, the Perry-Romney face-off illustrates the conflicting goals of many Republicans still shopping for a president. Many Republicans, and particularly tea partyers, remain wary of Romney largely because of his support for a health-care overhaul in Massachusetts that became the basis for the Obama plan.

But Romney’s business experience — and a formidable organization that he began building during his first run for president in 2008 — have attracted the support of plenty of Republicans, particularly those tied to the party establishment, who view him as best positioned to defeat Obama, a winner.

“Obviously, Governor Perry’s got to come in and introduce himself,” said Tim Pawlenty, the former Minnesota governor who dropped out of the race last month. “The polls would indicate that Mitt Romney does a little better against Obama. But this is a long road.”

At his debut debate appearance Wednesday at the Ronald Reagan Presidential Library in California, Perry scored points with the tea party by sticking to some of the more controversial pronouncements from his 2010 book “Fed Up!” — notably his view that Social Security is a “Ponzi scheme” under threat of imminent collapse.

Since then, Romney has been on the attack, seizing the opportunity to portray Perry as unelectable in a general election. Voters can expect Romney to stick with that portrayal in Monday’s debate and beyond.

“If we nominate someone who the Democrats could correctly characterize as being against Social Security, we would be obliterated as a party,” Romney said Thursday on Sean Hannity’s radio show.

Perry, meanwhile, will not back down, although a spokesman said the governor has no intention of abolishing Social Security. The issue provides a stark contrast with what the spokesman, Mark Miner, described as Romney’s “status quo” campaign.

“Social Security is a broken system that needs to be fixed, and anybody who says it doesn’t is doing a grave injustice to the citizens of this country and is just playing politics as usual,” Miner said. “You have people in this race who have different positions than they had three years ago, and the public’s tired of that. They want someone to talk honestly and openly about the issues.”

Romney and Perry aren’t the only candidates trying to navigate an impatient electorate. Bachmann announced Friday that she would concentrate her stalled campaign in Iowa, where her tea-party-friendly message has played very well, and where she is hoping for a badly needed boost in the first-in-the-nation Iowa caucuses next year. And former Utah governor Jon Huntsman, who is not conservative enough for many Republicans but argues that he is more electable than the rest of the field, announced that his campaign would shift resources to New Hampshire, where his moderation has a better chance of catching fire than elsewhere.

Romney is beset by his own set of weaknesses: To some tea party activists, his support within the party establishment is a mark of his business-as-usual approach to governing — and a sign that party insiders are still not listening to them. With a still-wide field of candidates including Bachmann, Cain, Rep. Ron Paul (Tex.), Huntsman, former House speaker Newt Gingrich (Ga.) and former senator Rick Santorum (Pa.), tea partyers don’t like being told whom to support.

Don Jakel, 60, a local activist with the Tea Party of West Michigan in Grand Rapids, said he and his associates are looking for a candidate who is “truly” a fiscal conservative committed to lowering spending, shrinking government and improving the atmosphere for business by eliminating regulations. Jakel is not a fan of Romney, whom he described as “closer to a liberal than a conservative” — but he also did not rule out supporting him, given Jakel’s overwhelming desire to defeat Obama.

“We certainly want to pick somebody who’s going to be electable, but I would prefer somebody who sticks with the Constitution a lot more than what any of the previous candidates or previous presidents have recently,” he said. “That’s what I’m going to be looking for. I don’t know if I’ll find it.”