WASHINGTON — As the Democratic presidential candidates pile on Education Secretary Betsy DeVos, one is facing new scrutiny over his past ties to her: Sen. Cory Booker of New Jersey.

Although Booker’s record as an advocate for public school reform has long been known, his past work with DeVos continues to dog him on the campaign trail as Democrats invoke her name with more and more frequency, turning her into their preferred bogeyman of the Trump Cabinet. The issue has put Booker, a former mayor of Newark, New Jersey, in the position of defending his past while candidates like Sens. Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont are emphasizing their pro-active plans on education and public schools.

Last week, while taking questions in Las Vegas, Booker was confronted by the vice president of the Milwaukee Teachers' Education Association, Ingrid Walker Henry, who accused him of diverting funding away from public schools for charter schools. She called him out for having “served with Betsy DeVos on the board of directors” of a DeVos-affiliated school choice organization.

“Will you say you’ve been wrong?” Walker Henry asked him at a forum hosted by the Working Families Party.

Booker pushed back, saying much of what has been said about his record on the issue is wrong. Of DeVos, he said he was “on the Senate floor fighting” against her nomination in 2017.

“I sat on a board with her with other Democrats looking for solutions, and I will never fault myself for trying to find solutions that work,” Booker said.

But from Iowa to New Hampshire and South Carolina, the 2020 candidates have been increasingly turning DeVos’ name into a proxy for incompetence and cronyism in the Trump administration. Although other Trump Cabinet chiefs had higher profiles before entering government, it’s DeVos who has attracted the most intense ire from Trump’s opponents.

Warren calls her the worst education secretary ever and says if she gets to pick the next Cabinet, DeVos “need not apply.” Beto O’Rourke called Friday to “replace Betsy DeVos” while Sanders likes to tell voters that in his administration, “we will no longer tolerate billionaires like Betsy DeVos.”

Mayor Pete Buttigieg’s standard attack on DeVos — that the U.S. needs an education secretary “who actually believes in public education” — has reliably become the biggest applause line of his entire stump speech. It’s such a crowd-pleaser that last week he launched Facebook ads accusing her of having “turned her back on lower-income and middle-class Americans.”

Booker, as a presidential candidate, has been sharply critical of DeVos. And when the Senate took up her nomination in 2017, he voted against her, citing her widely criticized performance in her confirmation hearing and her policies on special education and civil rights.

But Booker’s association with DeVos and the pro-school-choice organizations she had spearheaded actually dates back nearly two decades, to before Booker became mayor of Newark in 2002 and was serving on the City council.

A decade later, he was keynoting the annual conference for the DeVos-organized American Federation of Children, praising the group effusively as “friends of mine who were there for me in some of my darkest hours, people who have sustained me through their inspiration, people who instructed me through their example.”

“I kind of knew that this was going to be preaching to the choir,” Booker, then in his third term as mayor, said at the May 2012 dinner. “The folk in this room are just dedicated, many of you veterans in the effort to open up opportunities for all children in America.”

The unlikely alliance between Booker and DeVos emerged as both ascended nationally as champions of school choice and charter schools as alternatives to chronically underperforming public schools.

Booker took over a system in Newark so deeply troubled — and for so long — that the state had taken it over years earlier. A vocal advocate for school choice in those years, Booker teamed up with then-Gov. Chris Christie and Facebook founder Mark Zuckerberg, who donated $100 million to improve the schools in New Jersey’s largest city.

At the same time, DeVos and her husband, Amway heir Dick DeVos, were investing millions into national organizations to advocate for school choice policies across the country, including state laws and ballot measures to expand access to government vouchers to attend nonpublic schools. Those efforts elevated them as key enemies of teachers unions and other public school advocates who oppose what they describe as the draining of critical funds out of the public school system.

By Booker’s second term as mayor, he and DeVos were serving together on the board of directors of the Alliance for School Choice, a group closely affiliated with the American Federation for Children that DeVos later chaired.

And in 2013, when Booker won the Democratic primary in his first race for the Senate, the American Federation for Children put out a press release congratulating “our good friend Cory” and anointing him an “educational choice champion.”

Three years later he returned to the annual event for the American Federation for Children, with DeVos now serving as chairwoman. Video of his remarks reviewed by NBC News show he lavished praise on the group, although not DeVos by name.

“The mission of this organization is aligned with the mission of our nation,” Booker said.

He added that it was personal, saying it was “the first time I’ve been able to come back to this incredible organization since I’ve been elected to the United States Senate.”

“There are some people in this room who really were the difference makers as I was climbing the ladder in Newark, New Jersey, with a vision for transforming that city," Booker said.

Sabrina Singh, a spokesperson for Booker’s presidential campaign, said his efforts as mayor improved Newark’s schools and noted he’s been endorsed in the past by the New Jersey Education Association, the state’s largest teachers' union.

“Unquestionably, Newark’s kids are better off now than they were before Cory Booker ran toward the tough challenge of turning around the city’s failing schools,” Singh said in a statement.

Singh didn’t mention Booker’s past relationship with DeVos, but instead pointed to his refusal to support her as education secretary. “Cory vocally opposed the nomination of Betsy DeVos as Secretary of Education and has remained a strong critic of Secretary DeVos and the Trump administration's efforts to undermine public schools and public school educators,” Singh said.

DeVos declined to comment through a spokeswoman.

These days, Booker doesn’t regularly bring up charter schools or vouchers, controversial topics in a Democratic primary where other candidates are focused on raising up traditional public schools though additional federal funding, higher pay for teachers and expanded access to special education and early childhood programs.

Booker’s campaign website does not include any mention of the word “charter,” and his issues page focuses on the need to “pay teachers more, stop Republican attacks on public education, and invest in our schools.”