Every American president's worst re-election nightmare isn't an economy that's tanking as November approaches, or even having to face a popular opponent of the other party.
No, what presidents fear most is having to face a challenge to renomination from his own party. That's because having to defeat a primary opponent splits a president's base and certainly dooms his chances in the general election.
Ronald Reagan's 1976 challenge from the Right certainly sealed Jerry Ford's fate that November, while just four years later Democrat Ted Kennedy's campaign against Jimmy Carter assured Carter's miserable 1980 loss. Even conservative Pat Buchanan's 1992 GOP campaign mortally wounded President George H.W. Bush's chances for a second term.
Bill Clinton took this particular bit of history to heart leading up to his 1996 re-election, and assiduously discouraged any Democrat from challenging him on his way to a second term.
But, for Barack Obama, it already may be too late if Ed Rendell has anything to say about it.
The always-outspoken Democratic governor of Pennsylvania, Rendell raised the threat of a 2012 primary challenge against the president on the "Morning Joe" television program.
Specifically, Rendell warned of a Democratic challenge over Obama's escalation in Afghanistan, a policy which is unpopular with much of the Democratic base.
Rendell also poked Obama over the president's recent appearance on the daytime program, "The View," calling it undignified for the office.
It bears recalling that Rendell was one of the staunchest supporters of Hillary Clinton's 2008 campaign against Obama, and that the governor only endorsed Obama after Clinton tossed in her towel.
And Rendell's no local politician unexperienced in national politics. He's a former chairman of the Democratic National Committee and he understands how Washington politics works. That means he knows full well what he's doing by taking these kinds of shots at a president of his own party.