In 1864, Republican President Abraham Lincoln picked a pro-Union Democrat, Andrew Johnson, as his running mate. Johnson clashed continuously with the Republican Congress and became the first president to be impeached. In 1840, Whig President William Henry Harrison selected Democrat John Tyler for vice president. Tyler became president upon Harrison's death in 1841. Tyler found himself surrounded by old political enemies in a Whig Cabinet.
Those problems might be less serious for Lieberman should he quickly succeed to the presidency, however. He is on intimate terms with the McCain inner circle, especially Republican Sen. Lindsey Graham of South Carolina.
Still, Republicans assembling in St. Paul have their fingers crossed that McCain will not press his luck by naming Lieberman as his running mate.