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.