Democratic Sen. Tammy Duckworth is wading into the open primary to succeed longtime Sen. Dick Durbin, endorsing Lt. Gov. Juliana Stratton to join her in representing Illinois in the Senate.

Dick Durbin, the longtime second-ranking Democrat in the Senate, announced plans to retire at the end of next year, setting off a succession scramble to take his powerful leadership spot in the chamber and a likely crowded primary in Illinois for the seat.

Senator Richard J. Durbin’s departure will set off a primary for a rare open Senate seat in Illinois and open a top leadership slot. He said it was time to “face reality” and make way for someone new.