At a news conference Tuesday, Coleman says he won't accept a board's determination that Franken won 225 more votes in the November election.
The lawsuit will keep the seat vacant for weeks or months. State law prevents officials from issuing an election certificate until legal matters are resolved.
Coleman, whose term expired Saturday, led Franken by 215 votes in the Nov. 4 count, but the advantage flipped during a prolonged recount. Coleman's lawyers say recount inconsistencies and election irregularities should be reviewed by a special three-judge panel.
