Alabama Democratic Representative Parker Griffith is switching parties, citing differences with party leaders (particularly over health care) as the cause.
Obviously, Rep. Griffith has every right to do this (as did Senator James Jeffords, or Arlen Specter, or any other historical party switcher). Republican leaders, furthermore, are surely correct to trumpet the implications of a House member deciding to defect to the minority, which I imagine is a rarity. On the other hand, Matt Yglesias is also correct to note that Griffith came from a district whose Democratic control was essentially unsustainable over the long term. Rep. Griffith was the furthest thing from being a progressive while he was a Democrat; so progressives already understood that we would need to know how to push our agenda forward without him.