During a speech to 20,000 supporters in Manhattan Monday, Elizabeth Warren hailed Frances Perkins, who became a workers rights activist after the deadly 1911 Triangle Shirtwaist Factory fire.

But Perkins had a less admirable and less well known legacy later in life — as a critic of the Brown vs. Board of Education Supreme Court decision that outlawed segregation in public schools.

Americans, she said at the time of the 1954 decision, “should be nice to Negroes,” but not necessarily have to go to school with them.

And that left the Massachusetts Democratic senator in a position that her staff didn’t want to talk about following her own harsh criticism of fellow presidential candidate Joe Biden over his tone-deaf comments about being pals with a couple of segregationist southern senators.

“I’m not here to criticize other Democrats, but it’s never OK to celebrate segregationists. Never,” Warren sniffed in June after Biden’s gaffes, in which he cited his friendships with Mississippi Sen. James Eastland and Georgia Sen. Herman Talmadge as examples of how to work across the aisle in Congress.

During her speech in Washington Square Park Monday, Warren extolled Perkins’ activism after the factory fire, which killed 146 and injured 78, most of them women

“Frances pushed from the inside,” Warren said of Perkins, who later served as Labor secretary under President Franklin Delano Roosevelt.

“Frances Perkins became the first woman in history to serve in the cabinet. And what did she do when she got there? Big structural change.”

But Perkins held negative views of the landmark Supreme Court civil rights case, according to Steven White, an assistant professor of political science at Syracuse University, who cited her comments in his 2014 dissertation.

“Why, I think it was terrible. It was a purely political decision, and I think it should never have been made. I do indeed. I don’t know how they got it,” Perkins said of Brown v. Board of Ed.

“I mean, Earl Warren is a very diplomatic fellow, and he talked them into it. But there’s more to be said on it than they did,” Perkins said in an interview from the 1950s, referring to the then-chief justice

When the interviewer replied that it was “darned long overdue,” Perkins disagreed.

“No, it’s not overdue. It’s just begun to loom up as due — as nearly due. No wait! Nobody ever heard that segregation was wrong until about five years ago. I never heard such a thing. I never heard such a thing,” Perkins said.

“Certainly we should be nice to Negroes. Certainly we should treat them right,” the activist added.

Perkins died in 1965 at the age of 85, so she was in her late 70s or early 80s during that interview.

White tweeted Monday night that Perkins’ comments on race weren’t common knowledge.

“Regarding Warren’s use of Perkins in her speech tonight: I just want to note that this is something Perkins said near the end of her life, was buried in an extremely long academic oral history interview, and isn’t really public knowledge,” White wrote.

Indeed, researchers at the Frances Perkins Center were unfamiliar with the material.

White said Perkins wasn’t a “segregationist in the style of someone like James Eastland” — one of the Democratic senators Biden had referenced.

But he explained that Perkins disagreed with tactics used to push civil rights that were utilized in later decades.

“Even Harry Truman, who signed an executive order leading to the integration of the armed forces, was quite critical of the sit-in movement,” he said.

Reps for Warren’s campaign didn’t reply to a request for comment.

And the campaign continued to tout Perkins’ story Tuesday, using it in a fundraising email sent out to supporters.