The odds are surely against it, but as John McCain and Barack Obama near the conclusion of their vice presidential selection process, there's still some speculation that one, or both, might reach outside their party to pick their running mate. For Obama, the chatter has centered on retiring Republican Sen. Chuck Hagel of Nebraska; for McCain, the rumors have focused on Sen. Joe Lieberman, the "Independent Democrat" from Connecticut, who has been McCain's shadow on the campaign trail. Declarations from Hagel and Lieberman that they don't want the job haven't extinguished the discussions.

To the extent that the presumptive nominees are still considering these possibilities, history offers them three simple words of advice: Don't do it.

The president and vice president have been elected together since the approval of the 12th Amendment in 1804. (Before that, the candidate who attracted the most Electoral College votes won the presidency and the runner-up was chosen as vice president.) Since then, just two presidents have selected vice presidents who were not unambiguously members of the president's own party. Each instance proved an unconditional political disaster that effectively subverted the results of the previous election, tore apart the president's party and ended with efforts to impeach the man plucked from the opposition.