The former Bush ethics lawyer also had words for Sen. Tina Smith.
Richard Painter made the announcement on Wednesday.
Lt. Gov. Tina Smith will be sworn in a day later.
The Minnesota senator plans to step down next month.
Sen. Franken announced his resignation last week.
Trevor Noah, Seth Meyers and Jimmy Fallon mentioned the Minnesota senator's resignation.
Reactions from Congress, politicians, and notable Minnesotans.
It's forcing them to address the candidacy of Roy Moore in Alabama.