POTUS reelection is all about the economy and war. If the economy is doing well and we aren't bogged down in a war everyone hates defeating an in bent is really hard even if the Dems manage to find a decent candidate.
With a bad candidate (Kerry) POTUS can get reelected even when the economy isn't doing great and we're bogged down in a war everyone hates.
In bent Presidents are hard to beat. Actually the only one that has lost in my adult lifetime was H.W. and that took Perot splitting the vote to happen.