Clyburn said during a news conference Wednesday that a vote for fellow Democrat Alvin Greene would be an insult to his three daughters.
Greene is accused of showing pornographic images to a female University of South Carolina student. The unlikely nominee is an unemployed military veteran and political unknown who easily beat a party-backed former state lawmaker and judge in the June primary.
State party officials have asked him to step aside because of the indictment. Clyburn said state officials are suspended from office if they are indicted.
Phones messages left for Greene were not returned Wednesday.