Young and Clark offer two reasons:
First, a Republican will win because voters typically shy away from the party currently in power when an incumbent isn’t running. In fact, a successor candidate is three times less likely to win. Second, President Barack Obama’s approval ratings are too low to suggest a successor candidate will take the White House.There is reasoning behind each of these assertions, and they lay that out in some detail. They are by no means the first to posit this relationship, others have noticed that a non-incumbent member of the president's party is unlikely to be elected president.
This is particularly true when the president's approval ratings are below 55%. Barack Obama's approval has languished below 50% for most of his presidency, and is today about 45%.
