The Republican presidential field seems increasingly like some sort of poli-sci paradox: None of them can win the nomination but one of them has to. So what happens?

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Mr. Romney is already considered unacceptable by 41 percent of Republican voters and his favorability ratings have been fading recently. Although he is still okay for now, he does not have all that much margin to spare. It’s plausible that his ratings could turn negative if he runs a negative campaign, as other candidates begin to critique him, or if he makes one or two significant gaffes.

Mr. Gingrich has a fair amount of personal baggage and his favorability ratings were quite poor with Republican voters at points earlier in this cycle. He is now taking incoming fire from all sides, including from Mr. Romney, from other candidates like Ron Paul and portions of the conservative media.

But say that Mr. Gingrich survives this onslaught, as he certainly could. He wins Iowa and then either wins New Hampshire or finishes a respectable second there. He continues to lead in national polls of Republican voters, as he does now.

What happens then? One scenario, obviously — probably the most likely scenario — is that Mr. Gingrich wins South Carolina, and he wins Florida, and he holds on to win the Republican nomination.

But Mr. Gingrich might nevertheless be considered an unacceptable choice by much of the party establishment. That would put us in uncharted waters.

Silver doesn’t quite go so far as to say that it makes a brokered convention or a late-breaking establishment candidate likely, but I’m willing to go that far. There’s just no way the Republican establishment lets Gingrich become their nominee. As Andrew Sullivan pointed out today, you’re already seeing the anti-Gingrich mobilization among conservative thought leaders: Here’s George Will, Charles Krauthammer, David Brooks, Ross Douthat, Tom Coburn and Ann Coulter, just for starters. There’s this Politico story about all the Washington Republicans who hate Gingrich.

Now, I think it’s more likely that this mobilization leads to a Romney win then a brokered convention or a new entrant. But I think it makes a Gingrich win almost impossible to imagine. And if something once considered so impossible begins to look likely, then correspondingly extraordinary events — again, like a brokered convention — will become likely, also.