A former Republican White House lawyer and prominent critic of President Donald Trump is running for former Sen. Al Franken's seat. And he's running as a Democrat.

Richard Painter filed paperwork with the Federal Election Commission late last week to run for the seat after exploring a bid for more than a month. He is expected to formally announce his campaign Monday morning.

Painter served as chief White House ethics lawyer to President George W. Bush. He's emerged as a top Republican critic of Trump and is a frequent guest on MSNBC and CNN.

His entry as a Democrat triggers an endorsement fight and potentially a primary for Democratic Sen. Tina Smith, who was appointed to take Franken's seat after he resigned.