Elizabeth Warren, the liberal Massachusetts senator whose policy-oriented pitch to remake Washington and boost the middle class seemed to catch fire in the fall of 2019, ended her bid for the Democratic presidential nomination after a weak showing across a series of states over the past month.

Ms. Warren’s exit followed voting on Super Tuesday, when Democrats in more than a dozen states, territories and living abroad cast their ballots. She was unable to slow the progress of the leaders in the race, Bernie Sanders and Joe...