First it was Donald Trump. Then it was Michele Bachmann. Then it was Rick Perry. Then it was Herman Cain. Somewhere along the way, voters thought it might be Sarah Palin, or maybe Chris Christie.

Now it is Newt Gingrich.

We are talking, of course, about the anti-Romneys: the constantly churning field of Republican presidential candidates who, for a few days or weeks, are trumpeted as the definitive conservative challenger to Mitt Romney before falling back into the abyss.

As the allegations that he sexually harassed four women in the 1990s chip away at Cain's poll numbers, Gingrich is rising to take his place. A CBS News poll conducted last week showed Romney with 18 percent support and Cain and Gingrich tied with 15 percent, and subsequent polls from CNN and McClatchy showed Gingrich overtaking Cain for second place. He even made it to first place in a Public Policy Polling survey.

But Gingrich's chances of actually winning the nomination are slim. He has a tremendous amount of political and personal baggage, which will come into greater focus now that people are paying attention to him, and few people seem to be enthusiastic about a Gingrich presidency -- they are just turning to him now because there is no one else left to take up the anti-Romney mantle.

MSNBC host Chris Matthews devoted an episode of Hardball on Monday to Gingrich's newfound front-runnership and why it won't last.

I feel like we're in Wal-Mart, where they say, 'Shoppers, we have a special in Aisle 7, it's another non-Mitt Romney candidate, check it out!' But to win in politics, you still need money, organization, seriousness, and finally, to be able to survive scrutiny, David Corn, the Washington bureau chief for Mother Jones magazine, told Matthews. He is the beneficiary of this anti-Romney sentiment that has been conveyed with one candidate after another, but as soon as those candidates come into the spotlight and under the glare of scrutiny, they all wither.

Experience: A Double-Edged Sword

There are important differences between Gingrich and the anti-Romney candidates who preceded him, the biggest being experience. He has a longer political résumé than any other candidate in the race, having served as speaker of the House after engineering the 1994 Republican takeover, which could make him more appealing to conservative voters who are looking for a known quantity.

Gingrich is clearly a much more substantial, accomplished figure over a longer period of time than these other people who have been the candidate of the month, John Harris, the editor in chief of Politico, said on Hardball.

But that experience is a double-edged sword. Matthews noted that Gingrich is one of the career politicians that conservatives decry, and his long history in government has pluses and a lot of minuses, Harris said.

Gingrich's personal transgressions are well known: He proposed to his second wife while still married to his first and cheated on his second wife with his third, Callista, who was a congressional aide when their affair began.

His infidelity has already become an issue in the campaign, with the conservative group Iowans for Christian Leadership in Government distributing fliers that ask, If Newt Gingrich can't be faithful to his wife, how can we trust him to be faithful to conservative voters?

But more substantive are the questions that have been raised about his ideological consistency -- the very criticism that has dogged Romney throughout his campaign and led conservatives to seek an alternative.

In 2007, Gingrich said that individuals should not be able to cheat their neighbors by not buying insurance, particularly when they can afford it, and expect others to pay for their care when they need it. But this year, he blasted the individual mandate in President Barack Obama's health care law, saying he was against any effort to impose a federal mandate on anyone because it is fundamentally wrong.

In 2007, he praised the idea of a cap-and-trade program to combat global warming, calling it a package that's very, very good, and frankly ... something I would strongly support. But in 2008, he said cap-and-trade would lead to corruption, political favoritism, and would have a huge impact on the economy.

Gingrich has also changed his positions on climate change (it was happening, then it wasn't), on U.S. Rep. Paul Ryan's budget plan (he opposed it, then he supported it), on Libya (he supported U.S. intervention, then he didn't), and on trying suspected terrorists in American courts (he supported it, then he didn't).

In each case, he went from a moderate position to a position more palatable to conservatives -- exactly what Romney has been ridiculed for doing. Once conservatives start vetting Gingrich, as they vetted the potential anti-Romneys before him, they will find it difficult to ignore the double standard.

A Tough Road Ahead

To overcome the many marks on his record, Gingrich would have to put his track record into some kind of narrative of personal growth, Harris said. He has tried to do that by acknowledging, for example, that he has had moments in my life that I regret, but Harris said it would be a stretch for Gingrich to convince voters that he is a changed man.

I see in Gingrich's career more continuity than growth, he said. The Gingrich you see now doesn't look, to my eye, much different from the Gingrich who sprang upon the scene in the 1980s: very attack-driven, prone to disorganization, always interesting, but more surprises than most people want in a presidential nominee.

Conservative Republicans desperately want a viable candidate who will provide a rational way away from Obama, Matthews said. But Gingrich isn't that candidate, any more than Bachmann or Perry or Cain was.

I think they're looking for someone that can unite the conservative wing of the party and yet also be credible, Harris said. Michele Bachmann was interesting because of her conservative rhetoric that caused a lot of conservative hearts to go aflutter, but ultimately she didn't pass the credibility test, the plausibility test. They're looking for someone whose ideology resonates but who seems ready to carry the party's banner a year from now.

After a series of staff defections last spring, few people expected Gingrich to rebound at all, much less become a front-runner. He has proven those people wrong, and his recovery is remarkable.

But when the spotlight falls on him, he will not pass the credibility test. The reality is this: for better or for worse, he will fare the same as all the other conservative hares who have tried to unseat Mitt the Tortoise.