Few members of the Senate would have predicted a year ago that Bernie Sanders would be the last of five of their colleagues remaining in the final stretches of the presidential primary race.

Unlike Republican Sen. Ted Cruz of Texas, who returned to the chamber this week, or Sen. Marco Rubio of Florida, who ended his campaign in March, the independent from Vermont doesn’t seem to have planned years ahead for a White House run. When he entered the race last spring, he made clear his goal was largely to promote liberal policies...