Some Democrats have a nightmare that takes them back to Florida 16 years ago, and the time of the 'hanging chads'.
It was the presidential election decided in that state by 537 votes after weeks of counting, amid arguments over the ragged fragments of ballots not punched free in the voting machines. Those pesky chads.
The villain of the nightmare is the old consumer and green crusader Ralph Nader.
He persisted in his third party campaign through to November, impervious to Democrat accusations of selfish egocentricity, and got nearly 100,000 votes in Florida.
Election workers look over a ballot in Florida during the 2000 presidential election
With less than one per cent of votes for Nader, cast overwhelmingly by liberal-left voters, Al Gore would have won the state for the Democrats, and vote in the state-by-state electoral college.
President George W Bush would never have been.
The figure who hovers in this dream as a white-haired ghost is of course, Bernie Sanders.
Might he be the spoiler for Hillary Clinton which gives Donald Trump the White House?
The fear of senior Democrats is not that he makes a Nader-style independent run - it would make no sense at all - but simply that he poisons the well, and has the same effect in the end.
He's not giving up, although mathematically his chance of the nomination has gone.
He tells his huge rallies that the only way to defeat Trump is to defeat Clinton, and many of his followers believe him. I walked with a few hundred of them through San Diego.
They were a mixture of hardcore liberals, students, cragged hippies (wearing jeans that look as though they have seen service in '68), a man selling socialist pamphlets, and Aztec dancers who were asked to bless the march, which they did to a drumbeat that gave our microphone a few problems.
'Feel the Bern!' they cried.
It's a movement, without doubt. Fired by the spirit of the Occupy Wall Street campaign, and a belief that Hillary Clinton is the child of a rotten establishment, they sing the Sanders songs.
They are happy because, exactly like the Trump army on the other side, they are an insurgency which has surprised everyone.
But it perplexes Democrats who know how tough it will be when the campaign against Trump is truly joined.
Vice-chair of the California Democrats Eric Bauman told me that on 95% of the important questions, Clinton and Sanders held views that were nearly indistinguishable.
So why fight so hard and in increasingly fractious language?
California Senator and party elder Diane Feinstein has warned that they "can't afford a disruptive convention like 1968" (when anti war protestors were tear gassed in the streets of Chicago).
She was speaking after a state convention in Nevada where there was chair throwing in the course of a row about delegate selection.
I spoke to Stephanie Miller, the liberal talk-show host and comedienne who broadcasts from her home in the Hollywood hills.
She said that Karl Rove, the master Republican strategist had not bothered to turn his attention to Sanders yet.
"Imagine what he would do to a 73-year-old socialist Jew from Vermont!" she tells me, in the midst of a passionate assault on Trump, whom she described as "a racist, bigot and misogynist" and someone who reminded her of Hitler.
Yet Bernie soldiers on. He addresses vast rallies and he is tramping the valleys of California, scene of the last primary, in the hope of inflicting a final, embarrassing (though unlikely) defeat on Clinton, who's having to fight him with millions of dollars that she had hoped to keep for the campaign proper against Trump.
The nomination is all but locked up, but the internal party fight goes on.
Now, let's be clear that most of the Sanders voters in the primaries are bound to vote Democrat in November, whatever their feelings about candidate Clinton.
But how many won't?
And in supporting an increasingly sour Sanders attack on her, how far will they help to fuel the feelings of undecided voters who, for one reason or another stretching back 25 years, have never warmed to her?
His persistence is doing her damage, and some of it will last.
Such concerns seem far away to his supporters on the streets of San Diego. They continue to ask drivers to 'Honk for Bernie.'
Some of them said cheerfully that they could never vote for her. They would stay away on 1 November.
Donald Trump has already turned his attention to Hillary Clinton
And who knows, the polls may tighten as Republicans rally behind Trump, however reluctantly.
He's already beginning to tailor his message to try to pull in disaffected Democrats.
And if it works as it has done so far this year, some Sanders supporters will find themselves in November with walk-on parts in the Democrats' nightmare.