After Donald Trump nominated Betsy DeVos to become education secretary, teachers union honcho Randi Weingarten tweeted: “Trump has chosen the most ideological, anti-public ed nominee since the creation of the Dept of Education.” Since what’s good for the unions is often bad for the schools, and vice versa, Ms. Weingarten’s apoplexy is reason to cheer.

Ms. DeVos is chairwoman of the American Federation for Children, an organization dedicated to helping parents choose the best school for their kids. Ms. Weingarten leads the American Federation of Teachers, which is focused on what’s best for the adults.

Detractors say Ms. DeVos is opposed to public education. But she told an interviewer in 2013 that her definition of educational choice includes schools of all kinds. “What we are trying to do is tear down the mindset that assigns students to a school based solely on the zip code of their family’s home,” she said. “We think of the educational choice movement as involving many parts: vouchers and tax credits, certainly, but also virtual schools, magnet schools, homeschooling, and charter schools.” In the early 1990s, Ms. DeVos and her husband, a former president of Amway, were involved in passing Michigan’s first charter-school bill.

Ms. Weingarten brings a different set of priorities to the education debate. She has fought to keep persistently failing schools open because they still provide jobs for her dues-paying members. She has fought to ensure that government officials, rather than parents, decide where a child attends school. Union influence over education policy in the U.S. is unrivaled, and Ms. Weingarten prefers it that way. Her top concern is better pay and working conditions for her members. Students don’t pay union dues.

That doesn’t make her a bad person, but it should cast doubt on claims, too often swallowed whole by education reporters, that union interests are perfectly aligned with those of students and families. A union-negotiated work rule that says teachers can’t be evaluated by how much their students learn is a job-protection measure, but it obviously harms kids and school quality.

Education philanthropists often work to accommodate the teachers unions. Ms. DeVos chose to fight them head on by backing political candidates who support school choice, the same way unions support candidates who don’t.

Michael Petrilli, a veteran of George W. Bush’s Education Department who now runs the Thomas B. Fordham Institute, wrote last week that the DeVos pick shows Mr. Trump’s seriousness. “She was one of the first people in ed-reform to understand that we weren’t going to beat the teachers unions with op-eds and policy papers,” he wrote. “She pushed the private school choice movement to invest in serious political giving much earlier than the mainstream reform groups did, and, so far, with far greater success.” In the 2016 election, the American Federation for Children invested in 121 races in 12 states and won 89% of them.

Mr. Trump clearly has tapped a fighter, and education reformers are thrilled. The school voucher program in Washington, D.C., that President Obama has spent two terms working to shut down—at the urging of the unions, natch—is likely to flourish under the new administration. Mr. Obama and his Education Department supported charter schools but not vouchers. Ms. DeVos embraces school choice writ large, and states interested in expanding educational options for low-income families will proceed knowing that Washington has their back.

Mr. Obama tended to regulate what he couldn’t legislate, and education policy was no exception. The administration imposed its will from Washington in areas traditionally left to the states—from Common Core curriculum standards, to bathroom rules for transgender students, to race-based school discipline policies. With any luck Ms. DeVos will promptly end this meddling.

Reformers are also hoping that the Trump administration learns from the past. To the dismay of many conservatives, George W. Bush greatly expanded the role of the federal government in K-12 schooling through the No Child Left Behind Act. Insisting that school districts break down test results by subgroup—low income, special education, racial minorities—increased transparency. But rewarding and punishing school districts based on yearly progress was overreach that even some who supported the law now regret. It legitimized a more muscular role in education for the feds.

Mr. Trump has proposed a $20 billion federal voucher program that students could use to attend public or private schools. But this idea presents similar hazards. Federal dollars will bring federal regulations, and reform-minded individuals like Betsy DeVos won’t forever be in charge of implementing them. Better to let the states lead on school choice. Now that Republicans control 33 governorships and both legislative chambers in 32 states, what’s stopping them?