Six months ago, the Republican presidential field was looking robust, ranging from past candidates like Mitt Romney, high-profile GOPers like Sarah Palin, and party rising stars like Tim Pawlenty.  But as the months have progessed, the field has gotten considerably smaller.

In some cases, it's been scandal.  In others, its been performance.  In at least one other, it

After Barack Obama delivered his first address to a joint session of Congress back in March, Louisiana Governor Bobby Jindal- who had been considered an up-and-comer by many within the party- was tapped to deliver the GOP rebuttal.  After a performance that can only be described as a little goofy, Jindal fell off the face of the Republican world.  Buzz about the thirty-something conservative dwindled to nearly nothing, after having risen to a fevered-pitch in the months prior.

Jon Huntsman, the somewhat moderate Republican Governor of Utah, was a rising star and presidential contender in his own right.  Obama campaign chief David Plouffe cited Huntsman as the sole Republican candidate he'd least want to take on in a presidential matchup in 2012.  But shortly thereafter, Huntsman accepted a presidential appointment from his would-be opponent to become US Ambassador to China.  And while that doesn't necessarily preclude him from running for president in 2012, it would certainly make it difficult for him to run against a man of whom he thought highly enough to accept a political appointment.  Practically speaking, Huntsman is out of the running.

Then, John Ensign's admission last week that he had carried on an extra-marital affair for close to a year with a campaign staffer likely took the Nevada Senator out of contention as well.  In previous weeks, Ensign had emerged as a dark horse candidate, delivering a few key speeches in Iowa and opening a political action committee to raise funds for Republican candidates- both moves that could be considered steps toward a presidential bid.  But with his own personal indiscretions so recent, it's unlikely that he'd be able to clear his name in time for a national run.

Then there was Mark Sanford, who yesterday admitted to having disappeared for five days to trek to Argentina to meet a woman with whom he'd been having an affair.  Once considered a conservative darling, Sanford's star has rapidly faded, and is almost certain to fade even more in the coming weeks.

That leaves a few choice Republicans left waiting in the wings:

Newt Gingrich - The former Speaker of the House of Representatives may have all the government experience and conservative pedigree, but in essence, he's not that different from Ensign or Sanford.  Gingrich has been married three times.  He divorced his first wife literally while she was recovering in the hospital after surgery to treat uterine cancer, and married six months later.  He divorced his second wife after carrying on an affair (while he was investigating Bill Clinton's affair with Monica Lewinsky) with the woman who would become his third wife.  It will be interesting to see if Gingrich can get beyond his personal issues, because politically, he's a formidable force.

Sarah Palin - It's hard to figure what Sarah Palin has done that has not been controversial.  The pregnant daughter, the dismal attendance record as governor, the allegations that she cheated on her wife, the several hundred thousand dollar wardrobe on the GOP's dime, and the bevy of disastrous interviews.  Still, Palin remains one of the most well-known and popular Republicans in politics today, which makes her a top-flight contender.

Tim Pawlenty - The Minnesota Governor may be all that remains of the "rising star" class of Republican hopefuls.  With Jindal, Ensign, and Sanford potentially out of the running, Pawlenty may be one of the few lifelong, died-in-the-wool conservatives left to carry the ideological flag forward.  He's got credibility, experience, and a little national name recognition.  If he can stay scandal-free, he may very well end up with the nomination.

Mitt Romney - Mitt Romney likes to say that he, "hasn't always been a Ronald Reagan Republican.  But then again, neither was Ronald Reagan."  Romney, who had been the Governor of Massachusetts, one of the most liberal states in the Union, espoused somewhat socially tempered views while in office, only to take a hard-line conservative stance while running for president in 2008.  A little older, a little wiser, and an authority on conservative economic principles, Romney may be the man to watch.  In fact, while early polls favor Palin, the smart money would rest on Romney.

There are a few others who could end up making a serious run at the nomination and be successful: Sen. Jim DeMint of South Carolina, Sen. John Thune of South Dakota, Gov. Haley Barbour of Mississippi, and Governor Rick Perry of Texas have all been touted as possible candidates, though each have some issues they'll be forced to overcome.

But the field is, without a doubt, shrinking quickly.  Four top-tier candidates have been eliminated or badly damaged in the last six months.  What the next six months hold will be closely watched by Republican voters, as well as those who end up running.