Bernie Sanders, the progressive whose two campaigns for president pulled the Democratic Party to the left, ended his White House bid Wednesday, leaving former Vice President Joe Biden as the party’s presumptive nominee for the 2020 election.

The Vermont senator’s announcement created fresh urgency for Mr. Biden to unite the factions within the party and win over Mr. Sanders’s supporters. It also capped a remarkable turnaround for the former vice president and a stark fall for Mr. Sanders, who was the front-runner after the...