With fewer than 60 days until the Iowa caucuses, the struggle to become the conservative alternative to Mitt Romney is now the central dynamic of the Republican presidential campaign. Wednesday night’s debate in Michigan did little to clarify who will ultimately emerge to challenge the former Massachusetts governor, but it may have shown who will not.

Embattled businessman Herman Cain, who has denied allegations of sexual harassment, stood his ground on that and other issues when other politicians in his situation might have wobbled. He received a strong show of support from the audience at Oakland University. But the accusations remain a serious and potentially debilitating distraction to his bid and an opening for his rivals — if they can take advantage.

One of those contenders is Texas Gov. Rick Perry, who needed a breakout performance after seeing his candidacy decline in recent weeks. Instead, he froze onstage when he couldn’t remember one of the federal agencies — the Energy Department — that he would eliminate if he became president. All he could say as he struggled to recall was “oops.” The misstep is likely to be remembered as the most defining and damaging moment of his campaign.

Former House speaker Newt Gingrich (Ga)., who is rising in the polls and could be positioned to take advantage of his rivals’ weaknesses, spent part of the evening dishing out withering criticism of the media, as he has done in the past. Rather than offering a cheerful face to complement his obvious knowledge of the issues, he ended up in a verbal tussle with CNBC’s Maria Bartiromo, one of the moderators, who refused to be intimidated.

None of the others on the stage — Rep. Michele Bachmann (Minn.), Rep. Ron Paul (Tex.), former senator Rick Santorum (Pa.) or former Utah governor Jon Huntsman Jr. — made a memorable impression in this ninth GOP debate, which was co-sponsored by CNBC and the Michigan Republican Party. More than in any recent forums, the candidates largely chose not to challenge one another, a sign that they fear that attacks will be as damaging to them as to those they go after.

A vacuum on the right has become one of the distinguishing features of the campaign for the GOP nomination. One by one, candidates have come calling for support. One by one, they have stumbled or have been found wanting by rank-and-file Republicans.

First was Bachmann, whose rise was as surprising as it was short. Then came Perry, who zoomed to the top of the field and stayed there until Republicans saw him on the debate stage with the other candidates. His decline opened a path for Cain, who seized the opportunity and has remained at or near the top of the group since.

Republican strategists see two factors at work. First are the shifting sentiments of tea party supporters and others who define themselves as part of the most conservative wing of the party. Whether anyone can coalesce support among that part of the GOP electorate is now in doubt.

But these GOP strategists, speaking on the condition of anonymity to offer candid opinions about their party, also cite the relative weakness of the contenders. “You’ve actually got some candidates with accomplishments and credentials,” one strategist said. “But it seems like they all have deficiencies or vulnerabilities.”

The ebb and flow of those on the right has been as significant as the other reality of the GOP race, which has been Romney’s inability to gain additional support when one of his rivals has stumbled. His relative flat line in the polls, with somewhere between 25 percent and 30 percent support, has been seen as a sign of weakness.

But there is every bit as much weakness, if not more, among those who seek to challenge him.

Much now hinges on Iowa, where the competition is considered wide open. The winner there, if it is not Romney, will become the top challenger to the former Massachusetts governor, who is expected now to win New Hampshire the following week. A third candidate might also remain viable after that, but history shows that no more than three are likely to reach the finals.

In every Republican race in the modern era, the nomination has come down to a contest between the winners of Iowa and New Hampshire. Given Romney’s strength in New Hampshire, impressing voters in Iowa should be the goal of those trying to become his conservative challenger.

Cain has risen there as elsewhere, but until the sexual harassment story plays out, no one can say with any certainty whether he will be in a position by late December to win the state. His performance Wednesday probably cheered his supporters, but he has many difficult days ahead.

Whether Perry can recover from his debate mistake is questionable, given the problems he already had created for himself. His embarrassment was palpable, and he made an effort to repair the damage by appearing personally in the media filing center after the forum. “I stepped in it,” he said.

Gingrich could profit from Perry’s mistakes or Cain’s travails, but he carries plenty of baggage from his long career that could weigh him down. Bachmann and Santorum have devoted many days to campaigning in Iowa but have little to show for it. Paul has his followers, but not enough at this point to become a serious contender in Iowa. Huntsman has never tried to become the favorite of conservatives.

So unsettled is Iowa that Romney still could steal a victory there, according to some GOP strategists, who believe that is his real strategy. If he did that, and assuming that he went on to win New Hampshire, he could put himself in position to wrap up the campaign without a real fight.

That would go against all the history of recent GOP campaigns, which is why many Republicans expect someone to emerge as a strong challenger. But the vacuum on the right could turn out to be the biggest gift that Romney never expected.