And then there were four.

Texas Gov. Rick Perry’s decision to quit the presidential race means that only four men — former Massachusetts governor Mitt Romney, former House Speaker Newt Gingrich, former Pennsylvania senator Rick Santorum and Texas Rep. Ron Paul — will take the stage tonight for their 16th(!) gathering. (Who would have bet that quartet would have comprised the “final four” just a few months ago? You have to love politics!)

The four podiums for Republican presidential candidates, from left, former Pennsylvania Sen. Rick Santorum; former Massachusetts Gov. Mitt Romney; former House Speaker Newt Gingrich; and Rep. Ron Paul, R-Texas, stand candidate-less, at the North Charleston Coliseum in Charleston, S.C., Thursday, Jan. 19, 2012, prior to Thursday's debate. (AP Photo/Charles Dharapak)

* Newt-mentum: Gingrich’s rise and fall and rise (again) in the Republican presidential race tracks exactly to his performances in debates. His strong showing Monday night triggered a(nother) Gingrich renaissance.

The question is whether he can put duplicate that performance again tonight, this time with a much larger target on his back (or front). Gingrich succeeds most in these debates when he is able to cast himself as the big thinking, smart guy on the stage. If he is dragged into extended back and forths with his fellow candidates, he can seem preachy and overly combative. (Remember his close-to-condescending reproaches of Minnesota Rep. Michele Bachmann in debates gone by?)

Gingrich seems certain to take more incoming from his rivals and the debate moderators tonight than he did on Monday. And, it could get very personal with the allegations of “open marriage” made by his second wife swirling. Can he stay big or will he get down and dirty?

* Mitt as attack dog: In the vast majority of the 154 debates (or so) we’ve had to date, Romney has done everything he can to avoid direct engagement with his rivals.

But if you think back to that time — it now seems like eons ago — in the early fall when Perry seemed like the clear Romney alternative, you will remember that the former Massachusetts governor did show some willingness to throw the first punch. (Who could forget the “Romney shoulder touch” during a debate in mid-October?)

Romney’s campaign has rolled out a seemingly endless line of surrogates over the past 48 hours to savage Gingrich as undisciplined and unreliable. (The campaign even put out a web video blasting the former House Speaker as an “unreliable leader”.)

How strongly — if at all — will Romney take that line of attack to Gingrich tonight? The more aggressive Romney is, the more worried he is about Gingrich overtaking him in South Carolina.

* Santorum as x factor: Santorum, who apparently actually won the Iowa caucuses, looks like the forgotten man in the runup to South Carolina. Polling suggests he is running a distant third behind Romney and Gingrich in the state. But he may actually be the most important person on the stage tonight.

Why? Because Santorum has a choice before him: does he go after Romney in hopes of effectively casting himself as the not-Romney in the field or does he savage Gingrich in an attempt to peel away the conservative support that the former House Speaker is currently winning?

In our mind, Santorum would be far better to follow the latter course since Gingrich is the candidate directly in front of him in South Carolina polls and he and the former Speaker are in direct competition for the “consensus conservative” mantle.

Of course, Santorum has shown a willingness to ally himself with Gingrich before. He’s never done the same with Romney.

Which path does Santorum take?

* Outsider vs insider: The two candidates who sought to lay claim to the “outsider” title — Perry and former Utah governor Jon Huntsman, who dropped from the race Monday — are out. So, which of the remaining four candidates effectively makes the case that they are the best choice for voters who crave an outsider to the political process?

With the exception of Paul, who has made a career as an outsider but has a decidedly finite support base, none of the candidates are obvious outsiders.

Gingrich served as Speaker of the House, the most insider of insider jobs. Santorum spent almost two decades in the House and Senate. Romney may tout his experience in the private sector but he has been running for office or serving in office since 1994.

What we know from poll after poll (after poll) is that the Republican primary electorate is looking for someone willing to shake up the status quo. And they believe that person should be someone with limited ties (at most) to the current party establishment.

It’s a lane none of the top three contenders fit easily into. But you can bet each of them will try to squeeze into it during the 120-minute debate.

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