Barack Obama now has negative coattails for candidates: an endorsement by him causes more voters to reject the candidate endorsed than support the endorsee.  Variety, the showbiz newspaper, refers to unpopular actors as "BO [Box Office] Poison." The President with the same initials is voting booth poison. Tom Jensen of Public Policy Polling notes:

PPP's most recent national survey found that while Obama had a positive approval rating at 48/47, only 33% of voters were more likely to vote for a candidate endorsed by him while 48% said support from Obama would make them less likely to vote for someone. That's because only 64% of voters who approve of the President say his endorsement would make them more inclined to vote for a candidate, but 91% who disapprove say Obama's support makes it less likely they would vote for one of his preferred candidates.

To put into perspective the perils of having Obama out on the campaign trail, consider the numbers in his home state of Illinois. Even there just 26% of voters say they'd be more inclined to back an Obama endorsed candidate while 40% say his support would be more likely to turn them against a candidate.