Read between the lines and the liberal blogs (Tapped, DailyKoz, etc.)...

They've never been very jazzed about Kerry the candidate. Sure they drool at the though of ABB (Anyone But Bush) but most held their nose and punched their primary ballot for "electability." In the beginning they were jazzed about Dean, some were jazzed about Clark, and coming out of Iowa most jumped on the Edwards bandwagon. Most never really warmed to Kerry, but as his nomination became inevitable they begrudgingly got on board hoping that some of the early liberalism of the primaries would permeate into the Kerry campaign. Fortunately for Kerry the opportunity to do some serious Bush hating trumped their reservations about a Kerry candidacy..

The selection of John Edwards as Kerry's running mate plays to the audience; an audience that in most cases would rather have had him leading the ticket.

Update: InstaPundit still holds out hope for a July surprise dropping of Dick Cheney. Sound too far fetched to be true?

The The History Of U.S. Senate notes two similar circumstances:

Two unlucky vice presidents, Hannibal Hamlin and Henry Wallace, were dropped from the ticket after their first term, only to see their successors become president months after taking office, when the assassination of Abraham Lincoln made Andrew Johnson president and the death of Franklin D. Roosevelt raised Harry Truman to the presidency. Similarly, when Spiro Agnew resigned, he was replaced under the Twenty-fifth Amendment by Gerald R. Ford, who became president when Richard M. Nixon resigned less than a year later.

Update 2: This is a funny Edwards cartoon.

Update 3: The Moderate Voice identifies some criteria for judging the overall success (or failure) of the addition of John Edwards to the ticket.