Democratic presidential candidate Sen. Bernie Sanders (I-Vt.) speaks at a rally, Monday, Aug. 10, 2015, at the Los Angeles Memorial Sports Arena in Los Angeles. (AP Photo/Ringo H.W. Chiu)

Bernie Sanders keeps getting asked, and keeps saying no. No, he will not run as an independent candidate for president.

“I made the promise that I would not, and I will keep that promise,” Sanders said in his most widely shared version of the answer. “The reason for that is I do not want to be responsible for electing some right-wing Republican to be president of the United States.”

That’s clear enough, so why does Sanders keep getting asked? The latest attempt to stoke a Sanders-for-spoiler story comes from Ed Klein, the author whose incredibly fake-sounding transcripts of what Bill and Hillary Clinton say to each other have propelled a series of Obama-era books to the bestseller list.

Klein’s sources always speak exclusively to him, and exclusively in the sort of Bond-villain language that human beings don't use. In “Bernie Sanders is poised to go third party route” -- a story published by the rigorous fact-checkers of EdwardKlein.com -- Klein quotes “one of Sanders’ top campaign officials” on a clear plan to split the Democratic vote.

“As soon as [Hillary] starts piling up delegates with the help of Wall Street money and her formidable ground operation, Bernie’s going to pull out and announce an independent run,” said (or “said”) the source (or “source”). "Bernie’s polling has shown that he has a tidal wave of support among people across the country who have never or seldom voted. They’ll come out for him and pull the Independent Party lever.”

Even leaving aside the strange locutions – “pull the Independent Party lever?” – there’s no chance that a Sanders campaign insider said this. That's because there is no such thing as “Bernie’s polling.”

“We do not have a pollster,” said Sanders spokesman Michael Briggs. “We have not done polling. Bernie has said over and over that he will not be a spoiler, that he will not run as a third party candidate.”

Klein’s whoa-if-true journalism sometimes breaks into the mainstream. The latest piece has not gotten there yet, though a Newsmax.com aggregation of it has been shared on Facebook more than 1,500 times. What matters is that the story won’t go away. It is a whack-a-mole that has gotten stuck to the hammer.

But why?

One reason is that Sanders’s history with third-party spoilage is not well-known. True, he won his first elective office, the mayoralty of Burlington, Vt., as an independent in a three-way race. Since then, Sanders has caucused with Democrats and worked to elect Democrats. He has flirted only briefly with fellow independents, and endorsed former policy adviser-turned Progressive Party politician Anthony Pollina in local runs.

But when only a Democrat or a Republican has had a credible chance of election, Sanders has worked to stop the Republican.

This was proven several times in the four presidential bids of Ralph Nader. Once, in 2000, Sanders introduced Nader at a speech in Vermont. Nader, he said, was “an old-fashioned guy who believes that maybe the ordinary people should be running this country rather than the multinational corporations.”

But Sanders endorsed Vice President Al Gore over Nader. Four years later, he was one of the first people to condemn Nader’s do-over independent bid. “Virtually the entire progressive movement is not going to be supportive of Nader,” he told the Associated Press.

Nader has always resented this. Sanders has never regretted it. A third-party national candidacy, he saw, was not a way to influence the debate from outside. It was a way to be asked, constantly, how he felt about spoiling things for the Democrats.

Lots of progressives were chastened by the Nader experience, as nothing that the anti-politician promised came true. He suggested that a Gore defeat would heighten the contradictions between the parties -- a very Leninist argument. Within two years, half of the Democratic Party voted for the Iraq war. He hoped that the Green Party would become a force in politics, before his reluctant 2004 campaign split the party and erased most of its Nader-won gains.

That's all a bit complicated. The second reason for the endless "third party?" noodling is the press's tendency to seek balance in stories between the two main parties. Donald Trump is roiling the Republican primary, with voters hoarsely cheering for an "outsider." Sanders is doing the same for the Democrats. Trump occasionally threatens to go third party if he is not afforded "respect." Thus, Sanders gets asked if he's do the same.

He won't. In six months, Sanders has gone from wondering how to make a campaign credible, to outpacing Hillary Clinton in the first primary state. "The latest public surveys show him leading in New Hampshire and gaining ground in Iowa and elsewhere," said Briggs, pointing to Public Policy Polling survey that showed him also beating Republicans in head-to-heads.

And he should know. He is "One of Sanders's top campaign officials."