Arlen as a Dem?

Kos makes the case.

Specter won reelection by the skin of his teeth in 2004 and the conditions are much worse for him this time around. He's become even more of an outcast within the GOP in recent years.

It is starting to remind me of the late 80s and early 90s when Southern Democrats started switching over to the GOP.

The American Spectator had a piece in 2004 on the party switchers:
Another factor is Democrats' ability to convince a Republican to join their ranks; historically, they've had a harder time getting lawmakers to cross over to their side of the aisle. In the last 50 years, 16 Democratic members of Congress have become Republicans; only five have done the reverse. Democrats-turned-Republicans in the Senate include Phil Gramm of Texas, Strom Thurmond of South Carolina and Lott (although the retiring Gramm and Thurmond will be gone next year). After the GOP won control of both houses of Congress in 1994, Sens. Ben Nighthorse Campbell of Colorado and Richard Shelby of Alabama became Republicans, as did five House members. The only recent Republican-turned-Democrat is Rep. Michael Forbes (R-N.Y.), who lost his seat one year after his 1999 defection.

As for Richard Shelby becoming a Republican....good riddance!