Sen. Bernie Sanders (Vt.) speaks at an organizing event for Hillary Clinton at Lebanon High School Sept. 5 in Lebanon, N.H. (Photo by Darren McCollester/Getty Images)

WARNER, N.H. — Before he stepped up to the microphone, Sen. Bernie Sanders (Vt.) could tell there would be skeptics in the audience. He was on board with electing Hillary Clinton. Rights and Democracy, the progressive grass-roots group that had booked a small town's park for a Labor Day rally, was not.

"Is she indicted yet?" yelled one spectator, referring to Clinton. Sanders didn't visibly react. Elizabeth Ropp, an acupuncturist who had organized early for Sanders, introduced him by criticizing an unnamed "PR firm in a gentrified neighborhood of Brooklyn" and saying the country needed "a progressive third party." Sanders applauded, very softly.

On his first campaign swing since the launch of his political group, Our Revolution, Sanders tackled his most immediate and inevitable task: talking the progressives who had backed him into backing Clinton. Never expected to be easy, it has become as challenging, in its own way, as the "revolution" Sanders waged through the primaries. Nearly two months after Sanders endorsed Clinton, perhaps 10 percent of his supporters say they'll reject Clinton, vote for a third-party candidate or cast a chaos-making vote for Donald Trump.

That worries Democrats, who, despite Clinton's strength in the horse race, do not see many votes to spare. In an average of polls collected by RealClearPolitics, Clinton leads Trump by 3.3 points in a two-way race, but leads by just 2.4 points in the race most voters will encounter on the ballot: Clinton, Trump, Libertarian Party nominee Gary Johnson and Green Party nominee Jill Stein. But in every poll that asks, by a much wider margin, voters believe that Clinton will defeat Trump in November.

That has combined with the major party nominees' historically high negative numbers to produce steady, unusually high numbers for third-party candidates. Johnson, who is running more as a pragmatist than a dogmatic Libertarian, is pulling equally from Trump and Clinton — a departure for a Libertarian candidate. In a new Washington Post-SurveyMonkey poll of all 50 states, Stein polls best — at 10 percent — in Sanders's Vermont. And both candidates have argued that votes for them will spoil an election that deserves to be spoiled.

When pressed, Sanders is less critical of third parties than many Democrats. On Sunday's episode of "Meet the Press," Sanders said that the 15 percent polling threshold for the presidential debates was "probably too high." But with Trump and Clinton scrambling the electoral map, Sanders has become part of an effort to tell progressives not to go third party. That message is unusually resonant in New Hampshire, one of two states where the vote for Ralph Nader's 2000 candidacy was larger than the margin by which George W. Bush defeated Al Gore. That year, Nader took 22,198 votes; Gore lost by 7,211 votes. Had Gore won the state, he would have reached 270 electoral votes and made the outcome in Florida irrelevant.

On Monday morning, before Sanders himself addressed a Labor Day breakfast in Manchester, Sen. Jeanne Shaheen (D-N.H.) spoke at length about the Nader experience.

"When Al Gore was running against George W. Bush, people said, 'Ah, there's no difference between them,' so people stayed home," Shaheen said. "Some voted for Ralph Nader. Now, I hear people saying, 'Oh, I may vote for Gary Johnson.' Or 'I might vote for Stein.' What happened in 2000 is that we got George W. Bush. We got the war in Iraq. We got the biggest tax cuts for high earners in our history. If you think there's no difference between Hillary Clinton and Donald Trump, think about 2000."

In 2004, traumatized by the Nader experience, Democrats waged a multi-front war to keep Nader in the shadows. It successfully sued him off the ballot in some states by challenging his ballot petitions. Howard Dean, that year's revolutionary Vermonter, engaged Nader in a debate at the National Press Club.

"We ought to unite behind a Democrat who is progressive — we can argue about how progressive," Dean said. "When your house is on fire, it's no time to fix the furniture."

Nader's vote collapsed nationally; in New Hampshire, his total fell to 4,479 as the Democrats eked out a 9,274-vote win. And Sanders, who has only ever been elected as an independent and who supported Vermont's Progressive Party in several of that state's races, has consistently criticized the idea of a spoiler challenge to Clinton.

"If you run outside of the Democratic Party, then what you’re doing — and you have to think hard about this — you’re not just running a race for president, you’re really running to build an entire political movement," Sanders told The Nation's John Nichols in one of several interviews that broached the subject. "In doing that, you would be taking votes away from the Democratic candidate and making it easier for some right-wing Republican to get elected — the Nader dilemma.”

Yet Sanders ran strongest with voters younger than 30, none of whom were eligible to vote in 2000 and many of whom have no memory of that election. In Warner, he came face to face with the opposition, represented by a dozen Stein supporters who held their signs high throughout his 40-minute speech.

In three speeches across Labor Day, Sanders very slowly built to his endorsement of Clinton. In Warner, he spoke for 32 minutes about the possibility of a "revolution" on economic fairness and environmental protection, before finally pivoting to the election.

"I think that at this particular moment in history, as the senator from Vermont and as the father of seven beautiful grandchildren, that Donald Trump would be a disaster for the future of this country," Sanders said. "I am going to do everything I can to see that he does not win. And I am going to do everything I can to see that Hillary Clinton does win.

Most people in a crowd of about 250 cheered, but the protests from Stein voters were impossible to miss. They chanted, loud enough for him to hear: "Jill, not Hill! Jill, not Hill!"

"I understand it, okay?" Sanders said. "You are talking to the longest-serving independent in the United States Congress. I know a little bit about third-party politics. I understand that there are people who might not agree with me, and I respect that."

"I trust you, Bernie!" yelled a supporter.

"The cornerstone of Trump's campaign is bigotry, is dividing us up, is trying to get one group of people to hate another group of people," Sanders said. "Remember, before he was a candidate for president, Trump was a leader of the so-called birther movement."

This one-two punch was the one Sanders had delivered when he endorsed Clinton — a slow, methodical, hopefully inspiring rundown of what could be achieved in a Democratic victory, and a plea to vote for Hillary Clinton. Kari Zwick, 48, said that she had supported Sanders and did not thrill at the thought of supporting Clinton, but that Sanders had made the sale.

"I think she'll do good for the country," Zwick said. "I just don't think she's revolutionary enough."