Friday’s YouGov survey shatters the caricature of Holocaust deniers as old, white, right-wingers in rural America. The poll exposes the left as home to those who refuse to believe the fate of 6 million Jews and illuminates Democratic reluctance to expose the antisemitism festering in their midst.

YouGov asked 1,500 adult Americans if they “agree or disagree” that “the Holocaust is a Myth.” Ten percent of Democrats agreed, double the number of Republicans. Seven percent of liberals and 6 percent of conservatives said the same.

In urban areas, bastions of Democrats, the “myth” number hit 14 percent, almost triple the five percent in rural GOP strongholds. Thirteen percent of Black Americans and 11 percent of Hispanics, key constituents of the left, deny the Holocaust, too, versus five percent of whites who skew Republican.

Among those who voted for President Biden, seven percent believe the Holocaust never happened, one point ahead of those who backed President Trump. Overall, 11 percent of Democrats deny the Holocaust, three points more than Republicans. Liberals, at eight percent, outpolled conservatives on denial by two points.

The results reflect an evolution from 1962 when President Reagan, Senator Goldwater — whose father was Jewish — and the publisher of National Review, William F. Buckley, met at Palm Beach, Florida, to strategize against radicalism.

At the Breakers resort, Reagan, Buckley, and Goldwater resolved to denounce the John Birch Society’s fringe beliefs. Their efforts are credited as beginning a purge of antisemitism from the conservative movement, and the YouGov results testify to their success.

Despite lawn signs declaring “Hate Has No Home Here,” now the left is infected with the oldest bigotry. It’s a thorny problem since Holocaust denial is strongest in the youths upon whom  Democratic hopes for future electoral victories lie.

Among voters between 18 and 29, who supported Mr. Biden 62-to-35 percent over Mr. Trump, 20 percent agreed that “the Holocaust is a myth.” Zero percent of those over 65 — who favored Mr. Trump by three points — said the same.

Apathy was on display, too. Twenty-six percent of those under 30 said they “neither agree nor disagree” that the Holocaust was a myth. Twenty percent of Democrats also straddled the fence compared to 13 percent of Republicans.

On whether “the Holocaust has been exaggerated,” seven percent of Mr. Biden’s 2020 voters agreed against six percent of Mr. Trump’s. The number was eight percent among liberals and six for conservatives.

Among voters under 30, some 23 percent believed the Holocaust was “exaggerated” compared to just two percent of those over 65. In 2020, a poll commissioned by the Conference on Jewish Material Claims Against Germany, documented the same trend.

Conducted by Schoen Cooperman Research, “the first-ever 50-state survey on Holocaust knowledge of American Millennials and Gen Z,” found that 53 percent of the 1,000 respondents didn’t know Nazi Germany had murdered six million Jews.

Almost 20 percent in deep-blue New York — the highest number of any state — believed “Jews caused the Holocaust.” Overall, 11 percent of young Americans blamed the victims for their fate and almost half couldn’t name any of the over 40,000 Nazi death camps and ghettos.

Twenty-three percent of young citizens in that 2020 survey believed the Holocaust was a myth, exaggeration, or were unsure. Twelve percent said they “had definitely not heard or didn’t think they had heard” about it.

“The results are both shocking and saddening,” wrote the president of the Conference, Gideon Taylor, in announcing the survey. He added that it “needs to serve as a wake-up call to us all, and as a road map of where government officials need to act.”

Three years later, with Jewish people under attack worldwide and Israel at war with Hamas, the numbers show America remains asleep, at least on the left. Mr. Biden and Democrats seem hesitant to confront the problem.

Ignorance ignored, however, has a way of spreading. The right purged anti-Semitism decades ago. Now it’s time for the left to do the same. Only by facing the problem head-on can they live up to their lawn signs and ensure that hate finds no home in their ranks.