Clinton, Obama, Giuliani, Thompson

Some interesting new Gallup polling data of potential match-ups between four of the top tier candidates on both sides:

Clinton 44%, Giuliani 50%
Clinton 47%, Thompson 45%
Obama 52%, Giuliani 43%
Obama 56%, Thompson 36%

The poll also finds that Giuliani's unfavorables have gone from 22 to 32 percent in the last six months. Obama's have traveled a more modest 21 to 28 percent. Rudy's favorables have gone backward the more people know him - from 65 percent to 60 in six months. Obama's have only increased: 46 to 59 percent.   So in six months, Obama has closed a 20 point favorable differential gap on Giuliani. Remind me why any Democrat would back Clinton?