WASHINGTON -- As mayor of Newark, Cory Booker joined Betsy DeVos on the board of Alliance for School Choice, which advocated using taxpayer dollars for charter, private and religious schools.

He's known her for years.

But when DeVos was nominated to be U.S. secretary of education by President Donald Trump, Booker (D-N.J.) voted no.

DeVos was confirmed Tuesday, 51-50, when Vice President Mike Pence cast the deciding vote in favor of her nomination after two Senate Republicans joined Booker, U.S. Sen. Robert Menendez (D-N.J.) and every other Senate Democrat in opposing her.

"I found many of her answers inadequate, unsatisfactory ... or just plain violative of what I believe a leader in that position should do," said Booker, who has taken a more visible role in opposing Trump and broke with tradition in testifying against the nomination of fellow U.S. Sen. Jeff Sessions (R-Ala.) to be attorney general.

Despite their past history of working together, DeVos refused to meet with him, Booker said.

He said her hearing was rushed though so she couldn't properly be questioned, and was troubled by the answers she did give, such as refusing to rule out allowing guns in schools "to protect from potential grizzlies," and saying the federal Individuals with Disabilities Education Act, which is supposed to provide an appropriate education to students with disabilities, should be enforced by states.

"There are numerous things in her testimony that I found problematic and in totality made it not even a question for me whether i was going to support her," he said.

Booker was a champion of the school choice movement as mayor of Newark, looking for alternatives to the traditional public school system where some inner-city students were struggling.

Such efforts were opposed by teachers' unions, who decried the exodus of taxpayer dollars from public schools. While some of that money would go to public charter schools, which could operate under different rules than traditional public schools, other taxpayer funds would be spent on tuition at religious and private schools and for-profit charters.

Booker's advocacy of school choice put him in conflict with the Newark Teachers Union, which opposed his 2010 re-election as mayor. The union backed one of his opponents, former county prosecutor Clifford Minor.

Union President John Abeigon said he was "kind of surprised" that Booker voted against DeVos.

"He's a strong advocate for school choice," Abeigon said. "We never saw him much as a supporter of traditional public schools and don't see him as one now."

One-time allies said they were disappointed in Booker's vote against DeVos.

"He's turned into a partisan political player," said Peter Denton, founder and a trustee of the Clark-based school choice advocacy group, Excellent Education for Everyone, which Booker worked with. "It's extraordinarily disappointing."

Booker was the Orthodox Union's dinner speaker in Washington in September, where the senator talked about school choice. That's a big issue in the Orthodox Jewish community, said Nathan Diament, the group's executive director for public policy.

"He had a record as mayor of Newark supporting school choice initiatives," Diament said. "Most of our children attend non-public schools and the No. 1 kitchen table issue is being able to afford those schools. We reached out to Senator Booker's office, many of our constituents reached out to Senator Booker's office hoping he would support DeVos but he said he won't."

Booker had other concerns in deciding how to vote, said Krista Jenkins, a professor of political science at Fairleigh Dickenson University and director of its PublicMind polls.

"There has been some backlash against Democratic senators who have not opposed nominees in the Trump administration," Jenkins said. "I would not understand this as a wholesale rejection of his embrace of vouchers. This is a broad rejection of the Trump administration and the type of people he's trying to put together in government."

As someone mentioned as a potential 2020 Democratic presidential nominee, Booker can't afford to alienate the powerful teachers' unions, said Rogan Kersh, a professor of political science and provost at Wake Forest University.

The National Education Association, whose affiliates include the New Jersey Education Association, and the American Federation of Teachers, the parent of the Newark Teachers Union, gave more than $4.5 million between them to the super political action committee backing Hillary Clinton's 2016 presidential run, according to the Center for Responsive Politics, a Washington-based research group.

"A combination of unified Democratic opposition to DeVos and vehement disapproval by national teachers' unions -- long essential to Democratic presidential contenders' path to the nomination -- make a yes vote by Senator Booker effectively impossible," Kersh said.

Booker said he wasn't thinking about the next election.

"Politics be damned," he said. "It's about doing the right thing."

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant. Find NJ.com Politics on Facebook