The story from the White House is that they simply sent Bill Clinton to see if Joe Sestak was serious about running for Senate.
Well, buzz about him as a possible Senate candidate had been around since the preceding November. The Los Angeles Times, November 26, 2008:
The Northeast’s dwindling cast of Senate Republicans has Democrats circling Arlen Specter’s seat in Pennsylvania, convinced the party is well-positioned to make a competitive race out of the 2010 election . . . Others considered in the mix include Rep. Joe Sestak, who is sitting on $3 million in campaign funds.
So we’re to believe that at some point, Rahm Emanuel said, “Hey, there’s a two-term Democratic congressman who’s sitting on $3 million and who has been rumored to be interested in running for Senate since last year who’s making noises about taking on a cranky 80-year-old five-term incumbent whose popularity is tanking. Hey, let’s send a former president to see if he’s bluffing.”