John Cornyn threatens Republican Senators who oppose the earmark bans with right-wing primary challengers. Kevin Drum notes:

John Cornyn is the chairman of the National Republican Senatorial Committee. Can you even imagine his Democratic counterpart saying something like that? Any Democratic counterpart? And no, alternate universes don't count.

It's very easy to fall into the trap of assuming the other party is well-organized and clever while your party is disorganized and weak. But even while maintaining a healthy suspicion of that instinct, it seems to me that Drum is right. Until very recently, even the threat of right-wing primary challengers supported by pressure groups outside the party was rare. Now you have a member of the party leadership explicitly using this as a tool to enforce party discipline. At the moment it is completely impossible to imagine leading Democrats doing the same.

Which is to say, the Republican Party really is more disciplined than the Democratic Party. In part, this reflects the fact that the former is a straight conservative party while the latter is an amalgamation of moderates and liberals. Likewise, the Democrats' economic base is split between business, labor, and environmentalist or consumer groups, while the Republican economic base is purely business.

But I think it also reflects the fact that the Senate (and, to a lesser degree, the House) is tilted toward Republican control. In 2000, a year when the parties were very close to parity, George W. Bush won 30 states to Al Gore's 20. As a result, Democrats have a far greater need than Republicans to move to the center in order to be competitive.