Senator (and former Al Gore running mate) Joe Lieberman has been very supportive of John McCain, even going as far as forming a "Citizens for McCain" group. Some have even speculated about the possibility of McCain choosing Lieberman as his running mate, even though Lieberman is (kinda) a Democrat.
Could it happen? Probably not. But it is an intriguing possibility, especially if Obama starts to put some distance between him and McCain in the polls this summer.Selecting Lieberman as a running mate could strengthen McCain's support among independents. It would also hurt Obama's ability to portray him as another Bushian Republican.
Of course, it could just as easily blow up in McCain's face to tab a pro-choice Democrat for his running mate, but he should definitely keep Lieberman in mind.