In early primaries this year, he also failed to show that he had remedied a crucial weakness from his 2016 run: a lack of support from black voters, a vital base of the Democratic Party. In state after state across the South — Alabama, the Carolinas, Mississippi, Virginia — he was unable to chip away at Mr. Biden’s strong support among African-Americans.
In many ways, Mr. Sanders never overcame the widely held view among Democrats that he was a political outlier, a self-described democratic socialist who proudly proclaimed himself to be an independent senator from Vermont rather than a member of the party establishment.
Mr. Sanders championed liberal policies like “Medicare for all” and tuition-free four-year public colleges aimed at lifting up America’s working class, but he faced opposition from many party leaders, elected officials and major donors, as well as large numbers of moderate voters who saw him as too far left.
Mr. Sanders never accepted that argument. In recent weeks he said repeatedly that he had won the ideological debate, asserting that a strong majority of Democrats supported his progressive agenda.
But during a striking news conference in Burlington, Vt., last month, he also acknowledged that he was losing the electability battle to Mr. Biden, saying voters had made clear that they thought the former vice president was the best candidate to beat Mr. Trump.
The president immediately tried to sow discord among Democrats. In a Twitter post he blamed Mr. Sanders’s inability to win Super Tuesday states on his ideological rival, Senator Elizabeth Warren, and then invited Sanders supporters to “come to the Republican Party.’’ At his evening news briefing Mr. Trump was more pointed toward Mr. Biden, saying “It amazes me that President Obama hasn’t supported Sleepy Joe. It just hasn’t happened. When’s it going to happen?’’
Indeed, Mr. Trump’s penchant for no-holds-barred political combat presents another challenge for Mr. Biden. Some Democrats question whether he can withstand the kind of bitingly personal onslaught that Mr. Trump is certain to direct his way in the general election. The president’s efforts to tar Mr. Biden with the overseas business dealings of his son, Hunter, already upended the campaign once and led to Mr. Trump’s impeachment.