When the Supreme Court hears arguments in a case about whether parents can use state education funds for sectarian schools on Wednesday, the central question will be religious freedom versus separation of church and state.

But, just beneath the surface, there is a larger battle over school choice, including voucher programs, that could grow significantly depending on how the court rules.

The issue is a relatively unique program in Maine that provides education subsidies to rural districts that do not have their own high school. The state allows parents in that situation to use the money that would have been spent locally to send their children to other public or private schools – but not to religious education programs.

The parents of students who wanted to use the state subsidy for religious education filed a lawsuit, claiming that Maine’s policy violated their First Amendment right to practice religion without interference from the government. They filed their appeal at a time when the Supreme Court’s 6-3 conservative majority was favorable to religious freedom claims.

Some experts believe that if the Supreme Court rules in a way that requires states to fund religious schools in programs where they currently do not, the impact will extend far beyond Maine. For example, could states that allow charter schools, which are publicly funded but privately managed, be required to approve sectarian charters under the court’s ruling?

States that have voucher programs that allow parents to use taxpayer money to pay for private school tuition must already allow that money to flow to religious schools. However, supporters of educational choice argue that critics, including teachers unions and many Democrats, frequently raise the threat of litigation over religious issues in states considering such programs.

Experts say that determining whether such choice programs can fund religious schools without violating the Constitution’s prohibition on the government becoming involved in religion could take an arrow out of the quiver of those who oppose vouchers.

Leslie Hiner, vice president of legal affairs for the education advocacy organization EdChoice, agreed. Another issue, she claims, is that the way federal courts have defined religious schools gives states far too much power to scrutinize their curricula to determine whether they are “too religious” for state funding.

Teachers’ unions, including the National Education Association, and groups representing public school officials have reacted angrily to the possibility that the court’s decision will expand the use of vouchers or undermine states’ ability to set curricula for public education. Those organizations have traditionally opposed vouchers as a means of diverting taxpayer funds away from public schools, which they argue are in desperate need of funding.

School vouchers are one of the more contentious types of school choice programs because they allow taxpayer dollars to flow to private schools. According to EdChoice, voucher programs are currently available in 16 states and Washington, D.C. According to the group, they enroll more than 210,000 students, which is a relatively small number when compared to the approximately 5.8 million students who attend private schools.

Since 2010, voucher programs have grown significantly. From religious displays in public places to questions about the relationship between LGBTQ rights and religion, the Supreme Court’s conservative majority has sought to broaden religious freedom protections. That has also been true in education.

In 2017, the court ruled that a Missouri Lutheran church could apply for a competitive state grant to pay for playground resurfacing. In the majority opinion, Chief Justice John Roberts wrote that Missouri’s attempt to deny funding solely because the entity applying was a church was “odious to our Constitution” and “cannot stand.”

However, the court did not fully resolve the dispute, leaving unanswered a question about a subtle but important distinction: Lower federal courts have recognized a distinction between denying taxpayer money to an entity because of its status as a religious institution and withholding money used for a religious purpose, such as Bible teaching.

In the Maine case, the Boston-based appeals court concluded last year that the funding was not denied because the schools are Christian, but rather because of the Christianity the schools teach. Several of the high court’s conservative justices have questioned whether there’s really a difference between the two.