This has happened twice before. 1992 and 1912.
1912 was a huge fiasco in which Taft was denied the election when Teddy ran on 3rd party thus ensuring Wilson the election. He was pushed to run by the dems who basically poked a stick at him making him mad by reminding him that Taft was the GOP nominee, not him. Perot was urged by the Dems in much the same way in 1992 but on a lesser scale.