There was an interesting blogospheric exchange yesterday between Matt Yglesias and Nate Silver about the differences between the two parties' Senate Caucuses on disciplinary issues. Matt expressed a very common progressive envy for the willingness of Republicans to threaten serious sanctions for heterodoxy on key issues:

The Senate Republican caucus is organized, like the House caucuses of both parties, like a partisan political organization whose objective is to advance the shared policy objectives of the party. The Senate Democratic caucus, by contrast, is organized like a fun country club trying to recruit members. Join Team Democrat and Vote However You Want Without Consequence!

Nate, however, wonders how effective Republican Senate hardball tactics have been over the years in building a strong and loyal Caucus. Taking the 10 GOP senators deemed (by one credible measurement) most "liberal" in 2001, here's Nate's count of what's happened to them:

[Of the ten, there have been] two defections (Jeffords and Specter), two losses (Chafee and Smith), two retirements (Warner and Fitzgerald), two Senators that the party can pretty much no longer rely upon (Snowe and Collins), and finally, two who have indeed become more conservative and remained loyal to their party (McCain and Cochran). That's a .200 batting average, which isn't good in baseball and isn't any better in the Senate.

Perhaps it's just a coincidence that the size of the Senate Republican Caucus has also shrunk by 20% since 2001, but Nate makes the excellent point that enforced unity has its limits as a party-building exercise. Probably the best response to that kind of argument is simply that one joins a political party in the first place (particularly if you are a progressive) to get things done in the real world. If the "big tent" keeps thwarting that objective in fundamental ways, then its size is pretty much irrelevant.