Rep. Joe Sestak (D-PA) has released on official statement on the news that Sen. Arlen Specter has become a Democrat. To wit:

This shows the principle rule of politics: tomorrow is always another day -- as today was. This may be good for Arlen, politically; however, two key questions need to be answered. First, after 31 years in the military, I learned that you run for something, not against someone. Arlen has made a decision to leave a race because he could not win against someone. What needs to be known is what he is running for. Second, I watched then-Gov. Clinton and then-Sen. Obama take a leadership position in the Democratic Party and shape it. The leadership that would have been most impressive would be if Arlen had used his role to reshape the Republican Party that he said he had entered when it was a 'big tent,' but now is leaving because it has gotten too small. In short, I believe that the principles of what he is running for and his commitment to accountable leadership are questions that still need to be addressed.

Tomorrow's another day indeed. Specter's indicating that nothing will change about his politics now that he's a Democrat. It remains to be seen whether that will be reflected in his voting record from this point forward. But Sestak's statement is a warning that he oughtn't rest on his Republican laurels.