He didn't need anything that scientific. All he had to do was look at the primary returns for his district, then spend a moment and think about how it would look he voted for Clinton after the overwhelming majority of them voted for Obama. Politics is about personal survival. He'll vote his district. Nothing underhanded or disloyal about that. He owes the voters of his district far more than he ever owed either Clinton.
Obama having Jesse Jackson Jr, reminding them of that fact helped a lot to.