Paul Wellstone, one of the most principled liberals in the U.S. Senate, has died in a plane crash. His chartered plane went down in Evelethm, Minnesota, killing the senator, his wife, his daughter, three campaign staffers and two pilots. Wellstone was 58.

Aside from the awful human tragedy, this is a political nightmare, happening less than two weeks before an election. It’s like Mel Carnahan all over again. Wellstone was locked in a tough re-election battle against former St. Paul mayor Norm Coleman in a crucial race for control of the Senate. Larry Sabato recently labeled Wellstone “one of the most endangered Democratic incumbents in the country.” What will happen next is uncertain. CNN isn’t saying anything yet about whether/how he can be replaced on the ballot; the breaking news is too recent.