Efforts to use federal funding to finance private school education for military families could imperil the annual defense policy bill, Democratic members of the House Armed Services and the Education and the Workforce committees cautioned in a letter to those committees’ chairs Tuesday.

The National Defense Authorization Act (NDAA) “reauthorization is a critical piece of legislation that must pass on a bipartisan basis every year,” all 41 Democrats wrote in a letter to Armed Services Chairman Mac Thornberry (R-Texas) and Education and the Workforce Chairwoman Virginia Foxx (R-N.C.).

“The inclusion of any proposal that would send public resources to private schools is opposed by the Democratic caucus and could potentially derail any NDAA bill that contains such measure.”

At issue is a bill known as the Education Savings Accounts for Military Families Act that could be folded into the NDAA, which House Armed Services subcommittees are beginning to mark up this week.

The bill essentially would create a school voucher program by allowing military families to set up education savings accounts using funding from the $1.3 billion federal Impact Aid Program. The Impact Aid Program provides funding for schools and education programs on military bases, Native American reservations and other federal lands exempted from local tax rolls.

The idea has been championed by Education Secretary Betsy DeVos, who has said the accounts could be used by military families to ensure their children have a “customized” education as they move from “base to base to base and city to city to city.” The bill was sponsored by Rep. Jim Banks (R-Ind.).

In their letter, the Democrats highlighted that the Impact Aid Program was reauthorized in 2015 “by an overwhelming majority” as part of the Every Student Succeeds Act (ESSA). In the lead up to that act, the Education and the Workforce Committee held 69 hearings over the course of eight years, they added.

“Over those years, no one in either chamber suggested that Impact Aid should be privatized,” they wrote. “It is unfathomable that now, as ESSA is being implemented, you and your colleagues would be open to a major redistribution of Impact Aid dollars, a redistribution that would strip funding from 1,200 school districts and 10 million students.”

Military families were also “actively engaged” in that years-long process, the Democrats said.

“At no point did military families ask for Impact Aid dollars to flow to private schools,” they wrote. “Furthermore, other voucher proposals offered during ESSA consideration were soundly rejected.”

The Democrats urged Thornberry and Foxx to focus on other ways to help military families.

“Rather than dismantling this bipartisan program,” the letter said, “Congress should work to fully implement ESSA, monitor the educational achievement of military-connected students through the new subgroup data in ESSA, support the implementation of the Military Interstate Children’s Compact Commission, and focus on the support military families need, such as addressing TRICARE, providing opportunities for spousal employment and ensuring transitional benefits.”