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.

The retirement of Senate Minority Whip Dick Durbin (D-IL), the second-ranked Democrat in the upper chamber, is likely to usher in a new era of confrontation on the prominent Judiciary Committee. Political battles waged over the nation’s court system and presidential judicial nominees often begin on the panel.