Defying expectations, Arlen Specter’s week is getting worse. The latest poll shows Tom Ridge crushing Pat Toomey in the primary and beating Arlen Specter in the general election. (But Ridge isn’t yet in and it is, after all, only May of 2009.) Then Specter confided in an interview that he is rooting for the return of Republican Norm Coleman. Not surprisingly, the liberal blogosphere is going bonkers. Even before the Coleman comment Greg Sargent called Specter a “train wreck on the horizon.” But then comes the clincher. Coleman’s new Democratic colleagues voted to deny him the seniority promised by Harry Reid. Why does this matter? Well:

Without any assurance of seniority, Specter loses a major weapon in his campaign to win reelection in 2010: the ability to claim that his nearly 30 years of Senate service places him in key positions to benefit his constituents.

And then, to top it off, Specter took back his comment about Coleman.

A busy week indeed, which raises the question: is Specter trying to lose? If not, something is seriously awry and a Democratic challenge in the primary seems inevitable. Even if he loses that one, he could always run as an independent. But one senses Specter is becoming a punchline — and rivaling Chris Dodd as the most vulnerable Democratic incumbent.