As first reported by The New Republic, Pennsylvania U.S. Sen. Arlen Specter is joining the Democratic Party. Specter is pointing to GOP opposition to the economic stimulus bill, supported by the National Wildlife Federation, as the last straw:

When I supported the stimulus package, I knew that it would not be popular with the Republican Party. But, I saw the stimulus as necessary to lessen the risk of a far more serious recession than we are now experiencing. Since then, I have traveled the State, talked to Republican leaders and office-holders and my supporters and I have carefully examined public opinion. It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable.

What does Sen. Specter's switch mean for the chances of passing comprehensive clean energy & climate legislation in 2009? I'm going to go talk with some of our key legislative staffers and hope to have more on that this afternoon, check back later!