Why are Democrats Supporting Arlen Specter?

Arlen Specter, in his own words (emphasis mine):

At several other points, Specter did lay out areas in which he had "diverged materially from the Republican line," including raising the minimum wage, the stimulus package and abortion rights. But he went to great lengths to insist that he did not, as reported, tell Democratic leadership or the White House that he would be a loyal party member.

"I did not say," he told host David Gregory, "I would be a loyal Democrat. I did not say that."

The Democratic Party should not support this man's run for office. We should all support a challenger. Perhaps Joe Sestak?