What worries Democrats most about Bernie Sanders is what happens in the likely event he doesn't get the nomination.
The pundits, pros and party leaders admire how the Vermont senator has motivated so many people who had seemingly tuned out of politics, particularly young voters. He had been given him chance of grabbing the gold ring, but the party wants him and his followers to carry that enthusiasm into November.
Hillary Clinton has tried, not always successfully, to avoid alienating Sanders' followers because she knows how badly she will need them in the general election.
Sanders "has to decide whether he plays the Ralph Nader/2000 role and sabotages" Clinton, said veteran Washington analyst Chris Nelson.
Consumer advocate and Green Party candidate Ralph Nader ran a grudge campaign against Democrats in Florida. He knew he couldn't win, only take enough votes away from Al Gore in Florida to deny him victory and to hand the election to George W. Bush.
Something similar happened in 1968. Sen. Eugene McCarthy, the anti-war candidate, lost the Democratic nomination to his friend, mentor and fellow Minnesotan Hubert H. Humphrey. Instead of throwing his support behind the party's nominee, who had done so much for McCarthy's career, he discouraged his followers, many of them young, from voting that year. Playing the embittered spoiler, he helped elect Richard Nixon.
Sanders, like Nader and McCarthy, is a leftist icon who has energized many young and new voters. But Democrats are hoping that is where the similarity ends.
Clinton lost an often bitter battle for the nomination eight years ago to Illinois Senator Barack Obama, but she went to the convention to nominate and endorse her rival and worked hard in the coming months for his election.
Which example will Sanders follow? Nader and McCarthy or Clinton?
Sanders has said electing a Republican "would be a disaster for this country, and I will do everything I can to prevent that." But that won't mean anything unless he tells his ardent followers that, if he is not the candidate, they must work just as hard to elect Clinton.
If they stay home they will be robbing Bernie of whatever influence he might have gained in the next Congress or the next administration, and they will be electing Donald Trump or Ted Cruz.
Sanders has already made history as the first Jew to win presidential primaries and to be a serious contender for his party's nomination. That achievement would be tarnished if he were to follow the sour grapes examples of Ralph Nader and Eugene McCarthy.