WASHINGTON — Just as Sen. Bernie Sanders’ progressive presidential challenge to Hillary Clinton is gaining steam, an old essay of his has surfaced in which he says women fantasize “being raped by 3 men simultaneously.”

“A man goes home and masturbates his typical fantasy. A woman on her knees, a woman tied up, a woman abused,” Sanders, 73, wrote in 1972 for an alternative newspaper called the Vermont Freeman.

“A woman enjoys intercourse with her man — as she fantasizes being raped by 3 men simultaneously.”

In another part of the article he wrote:

“Do you know why the newspapers with articles like ‘Girl, 12, raped by 14 men’ sell so well? To what in us are they appealing?”

Sanders was 30 at the time and was running for both the US Senate and governor of Vermont on the leftist Liberty Union Party line.

Sanders’ campaign is now trying to distance him from the article, calling it a “dumb attempt at dark satire in an alternative publication intended to attack gender stereotypes in the 1970s” that in “no way reflects his views or record on women.”

“It looks as stupid today as it was then,” said spokesman Michael Briggs.

Briggs characterized the resurfacing of the essay — which was first reported by Mother Jones magazine — as an attempt to distract from the real issues.

Sanders is determined to run a campaign about big problems and not “salacious gossip and innuendo,” Briggs said.

Sanders has a record in the US Senate of backing bills to protect women, including sponsoring a no-tolerance policy for sexual violence in prisons and the renewal of the Violence Against Women Act.

But the perception of the essay could create trouble for him as he seeks to defeat Clinton, who coined the phrase “Women’s rights are human rights.”

A new Quinnipiac poll found Clinton leading the 2016 Democratic primary with 57 percent, but Sanders is second at 15 percent.