Bernie Sanders has said that he will support the Democratic Party's presidential nominee, no matter who he or she is. But some Democrats worry that a lot of his supporters will not work or even vote for any other candidate, whereas the backers of his Democratic rivals will enthusiastically work or vote for anyone the party nominates, including Sanders.
It is too soon to know whether this worry is justified. But we do know that in 2016, many of Sanders's supporters were extremely angry that Hillary Clinton won the Democratic nomination, and they refused to support her.
According to one estimate, over 20% of Sanders voters declined to vote for Clinton - and they may well have thrown the election to Donald Trump. The estimate found that as many as 12% of Sanders supporters actually voted for Trump, and an additional 10% or so stayed home or supported a third-party candidate.