Arlen Specter has always been a survivor. The Pennsylvania Senator has endured two bouts of Hodgkin's lymphoma and the chemotherapy that goes with it, a couple of procedures for a recurrent benign brain tumor, and heart-bypass surgery that sent him into cardiac arrest. And in a political career that has spanned 45 years, he has regularly sidestepped doom. Specter's most celebrated swerve came last April, when he switched parties to avoid a Republican primary against a conservative challenger he had barely beaten in 2004. He acknowledged he could never win the GOP nomination for a sixth term after voting for Barack Obama's economic stimulus package -- a move that was one heresy too many for the shrinking Pennsylvania Republican Party.

At the time, Specter's switch was hailed as a heady affirmation that Barack Obama had ushered the nation into a new, post-partisan era. With the defection of one of its last Senate moderates, what was left of the GOP appeared to be careering rightward, to a hard-core base that was beginning to resemble a cult as much as a political party.

But a year later, those calculations have been tossed upside down. Obama's poll numbers have come back to earth. And the filibuster-proof Senate majority that Specter's defection delivered to the Democrats vanished when Massachusetts voters handed Teddy Kennedy's old seat to Scott Brown. Democratic control of the House is in jeopardy, and the party stands to lose at least a half-dozen seats in the Senate. "Unless something significant changes," political handicapper Charlie Cook wrote last month, Democrats "are headed toward the losses of the magnitude we saw in the midterm elections of 1958, 1966, 1974, 1994 and 2006."

A Shifting Keystone