amtrakcabin.JPGNo one will ever accuse Joe Biden of lacking for enthusiasm. "Arlen Specter has been my friend and my confidant and my partner," he said today, using language more commonly associated with wedding proposals. "And it gives me great pleasure, great pleasure, Mr. President, to now officially be in the same caucus with Arlen Specter. We’ve ridden the train for so many years, we’ve visited each other’s homes, our families, that, as a point of personal privilege, it’s just a delight to have no separation."

Specter was similarly nostalgic. "We have talked over every problem under the sun and under the moon," he agreed. "We’ve ridden that train together again and again, and we’ve supported that train."

It all sounds pretty sexy. Barack Obama, as is his tendency, was more measured. His remarks were mainly a warning to Joe Sestak and the other Democrats who are considering challenging Specter in the primary. "Let me start off by just saying I’m thrilled to have Arlen in the Democratic caucus," said Obama. "I have told him that he will have my full support in the Democratic primary. Joe Biden has said the same thing. We are confident that Arlen Specter is going to get a 6th term in the Senate and the American people are going to be better off for it."

Maybe. There's one argument that the optimal outcome for Democrats is for Arlen Specter to fear, and then to lose, a primary challenge from a Pennsylvania Democrat with more grassroots support. In that world, Specter has an incentive to vote as a liberal until the primary and, after the election, he can be replaced with an actual liberal who will vote that way out of conviction. The counterargument is that if Specter is defeated in the coming primary, it will deter other vulnerable Republicans from following in his footsteps.

Either way, Obama ended the presser by fondly recalling those long hours Joe Biden spent with Arlen Specter on the Amtrak. "I’m grateful that [Arlen] is here," said Obama. "And I’m also grateful that Joe Biden paid him a little attention on the train."

Full press conference transcript after the jump.