Over at the WaPo, Chris Cilizza writes of Tuesday's special election in PA-12:

For Republicans, the race is simply a must-win.

While there are arguments to be made about the large party registration edge Democrats carry in the 12th and the fact that the special will coincide with statewide primaries where Democrats have far more competitive races than Republicans, the fact is that this is just the sort of culturally conservative swing district that the GOP must prove it can win to take back the House this fall.

Cilizza severely downplays the counterarguments here, and overstates the stakes for Republicans. First, Republicans haven't won this district since the 1930s,[1] and Democratic registration vastly outstrips Republican registration (most of the district that originally elected Murtha has been dismembered and placed in neighboring districts). While McCain did carry it, he only did so by 900 votes.

To put it differently, there are over sixty districts represented by Democrats with better Republican performances than PA-12. The Republicans' path to 218 seats doesn't necessarily run through this district – in fact, I don't think their path to a 1994-esque 230 seats necessarily runs through this district.  A Democratic loss in this district tells me that Jacksonians really are abandoning the Democrats at the Congressional level, and all sorts of hell is about to break loose in November.

The path of least resistance for Republicans still runs through places like OH-01, NY-29, IL-11, and NM-02: historically Republican districts that voted out GOP incumbents as part of the anti-Republican wave of 2006, or the Obama wave of 2008. PA-12 is, quite frankly, gravy for Republicans.

Second, and perhaps more importantly, the danger for the Democrats in November stems in large part from the enthusiasm gap between Republicans and Democrats. Republicans are presently more enthusiastic about voting than Democrats; the most recent polling shows that the most enthusiastic voters prefer GOP control of Congress by twenty points. Needless to say, this is a big change from 2008; merely pulling even in enthusiasm would probably net the GOP a dozen seats or so by itself.

But the Democrats have a built-in advantage to combat this effect in PA-12 that won't be present in November: Highly competitive primaries for Senate and, to a lesser extent, Governor. The Senate race in particular seems likely to drive Democratic turnout, while Republicans have no such upticket race to assist with GOTV efforts. In other words, it wouldn't be surprising to see Critz win by 2 or 3 points this time, but lose in November.

I quite frankly have no idea what's going to happen next Tuesday. Special election polling is notoriously unreliable, precisely because it is difficult to account for things like the Specter-Sestak primary driving downballot turnout.  Because of this, a Democratic win doesn't tell us much about November, but a Republican win might.

[1] The current 12th is an amalgamation of three districts that existed at the time that Murtha was elected: the 12th , the 21st and the 22nd. The 21st and 22nd districts didn't elect a Republican post-1932, and the portions of those districts contained in the current iteration of the 12th cast almost 2/3 of the vote in the present district. By contrast, the portions of the old 12th (which usually elected Republicans) still in the current district account for only about 1/3 of the district's vote.

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