It has happened four times (Franklin Pierce, Millard Fillmore, Andrew Johnson, and Chester A. Arthur), but three of them weren't elected president in the first place (got there via deaths) and all of them happened before party primaries were decided by voters.

http://wiki.answers.com/Q/Has_an_incumbent_president_ever_lo...

> Is there precedent for a party to put up a different choice in the primary when there's a reigning incumbent?

The parties elect their candidates in a primary before putting them on the ballot in the general election. But parties don't like primaries: You have two of your own candidates saying bad things about each other in public, taking specific stances necessary to win a primary that may be disadvantageous in the general election, and using campaign money that could otherwise have been used against the other party in the general election.

So the result is that if the incumbent wants to run again, generally no one from the same party will be willing to mount a primary challenge, and anyone who does will raise the ire of other party members for causing trouble and wasting money. Hence the absurdly high incumbency rate.

That's exactly what I would have expected, which is why, I suppose, that it hasn't ever happened since primary voting became open to full registered party members.

Joe Lieberman lost the Democratic Party's support in his 2006 senatorial race by not being a Democrat. He ran as an independent and won.

http://en.wikipedia.org/wiki/Joe_Lieberman#2006_Senate_elect...