Jim Geraghty gives his analysis:

Beyond that, McCain would be asking Republican voters to put a socially and economically liberal Democrat a heartbeat away from the presidency, said heartbeat belonging to a 72-year-old who has survived cancer. A lot of Republicans and conservatives like Joe Lieberman, but they don't want to see him as president.

Ridge was last elected to high office, the governorship of Pennsylvania, in 1998. Beyond his work with the Hershey Corporation, he hasn't spent an enormous amount of time in the Keystone State since leaving to run homeland security in late 2001. It's not fair, but he's probably most associated with the color-coded terror alert system instituted at the Department of Homeland Security. Is Ridge worth 4 to 12 percentage points in Pennsylvania? He would likely help McCain, but not by that much.