At noon on Tuesday, the Senate is expected to vote whether to confirm Trump's controversial nominee for education secretary, billionaire Betsy DeVos, following an unusually fiery debate including an all-night Senate session, that has left her at risk of becoming the first cabinet nominee in 28 years to be rejected. Although Senate Republicans and the White House have said they are confident they have the votes to confirm Mrs. DeVos as the nation’s top education official, the vote may be on the verge of failing, and the GOP might need to take the rare step of turning to Vice President Mike Pence to cast a tiebreaking vote as the WSJ reports.

The troubling math: cabinet picks need a simple majority to be confirmed. Republicans hold 52 of the Senate’s 100 seats, but two—Lisa Murkowski of Alaska and Susan Collins of Maine—say they will vote no. DeVos would win confirmation if all other Republicans and Mike Pence vote yes.

While it is unusual for a presidential-cabinet pick, let alone an education-secretary nominee, to face the kind of opposition Ms. DeVos has faced, her nomination has become a proxy for a broader debate over how best to improve public schools across the U.S. Over the past month, teachers unions and Senate Democrats have voiced their opposition to DeVos’s decades of leadership in the “school choice” movement, which advocates charter schools and voucher-funded private schools as alternatives to traditional public schools. Charter schools are publicly funded but mostly privately run, and vouchers provide public dollars for students to attend private schools.

More details from the WSJ:

Critics of Ms. DeVos, a 59-year-old billionaire from Michigan, say the movement redirects money that otherwise would be used to improve public schools. Students who remain in public schools end up worse off, they argue, and they say many charters don’t offer better educations. They have also pointed to statements Ms. DeVos made during her January confirmation hearing they say show she has a weak grasp of education policy and is unfit for the job. For example, Ms. DeVos appeared confused about the Individuals with Disabilities Education Act, a U.S. law that ensures students with a disability receive an education tied to their needs.

Perhaps more concerning to her critics, is that DeVos has said previously that while she has been a part of organizations that supported Common Core, she is not a supporter. And even though the US graduation rate increased to a record 83% in 2014-15, the most recent year available, from 79% in 2010-11, many are skeptical about the aptitude of thoe graduating.

DeVos' supporters say public-school students in low-performing schools should have other options, just as wealthy families who send their children to private schools do. They say charter schools often use creative teaching methods and have more flexibility than public schools to fire bad teachers.

The U.S. has stepped up education spending for the lowest-performing schools with mixed results. Under the administration of former President Barack Obama, billions of dollars in grants have targeted struggling schools and charter enrollment has grown. The administration also used competitive grants to nudge states to adopt Common Core, academic standards that outlined what students should know in reading and math. But reading and math scores were essentially flat on the most recent Nation’s Report Card, a measure of student achievement.

Data show the U.S. ranks among the highest in the world in education spending. In 2012, the U.S. spent $11,700 per full-time equivalent student on elementary and secondary education. That was 31% above the average of countries in the Organization for Economic Cooperation and Development, according to an Education Department report last summer citing OECD data.