Arlen Specter’s party switch isn’t all that startling. Richard Shelby and Ben Nighthorse Campbell switched to the Republicans right after the 1994 election, without (as far as I know) facing the same kind of primary challenge. But this switch is especially important, because once Al Franken finally gets seated it will give the Democrats the magic 60 number. The way is now open to a seriously progressive agenda.

What strikes me, however, is the extent to which this is a self-inflicted wound. If Pat Toomey of the Club for Growth weren’t so diligent about enforcing supply-side purity; if Republicans hadn’t made Rush Limbaugh the effective head of the party; Specter might still be GOP, and the Obama agenda much more limited.

Instead, though, we have a party that seems to be in a death spiral: the smaller it gets, the more it’s dominated by the hard right, which makes it even smaller. In the long run, this is not good for American democracy– we really do need two major parties in competition. But I’ll settle for getting that back after we get universal health care and cap-and-trade.