President-elect Donald Trump’s biggest education pitch during the presidential campaign was for a $20 billion voucher program that students could use at both public and private schools. So, now that he’s been elected, how feasible is that? And would states even be interested in it?

The money would have to come from somewhere. Trump said during the campaign that he’d like to use existing federal funds to support his big school choice program, even though he didn’t say, specifically what pot of money he was referring to. The department’s current budget is about $70 billion, with roughly $15.5 billion going to Title I grants for districts, and $12 billion going to state grants for special education.

Both programs have been absorbed into the blood stream of school district budgets, so re-directing the money would be a big deal.

“Twenty billion dollars is a lot of money,” said Vic Klatt, a principal at Penn Hill Group in post-election event sponsored by the Education Writers Association. “Finding that will be an interesting challenge for them. I’m not quite sure how they do it.”

And groups representing school districts, teachers, state officials and others would fight it hard. If enacted, the proposal would “undo decades of growth in student achievement, closing of achievement gaps, and growth in graduation rates,” said Noelle Ellerson Ng, the associate executive director of AASA, the School Superintendents Association.

Congress would have to amend the Every Student Succeeds Act or the Individuals with Disabilities Education Act. Trump couldn’t just snap his fingers and make the proposal a reality. He would have to go through Congress, which would have to pass the change by amending either ESSA or IDEA. That may not be easy. The Senate rejected a (somewhat similar) proposal from Sen. Lamar Alexander, R-Tenn., last year when Republicans controlled more seats than they will in the new Congress. Trump and his allies would have to put it way up at the top of their priority list—or significantly expand the GOP ranks in Congress—for it to have a chance of passing.

Allocating the money could be difficult. Title I dollars don’t work like Pell Grants for low-income college students. They are distributed by a complicated formula and the amount per child varies from state to state and district to district. So the mechanics of a so-called Title I portability proposal—if that’s what Trump is after—aren’t simple. What’s more, states and school districts would almost certainly have to kick in their own funding to make it workable.

Last year, when a similar idea was under consideration in Congress, Mike Griffith, a school finance expert who works with the Education Commission of the States, noted that Title I dollars are geared toward concentrations of poverty. So a student taking Title I money from a school with a lot of poor children to a school with fewer students in poverty might be given a smaller allotment.

In a more recent interview, Griffith said it was tough to tell at this point how feasible Trump’s proposal would be at the state level since no hard-and-fast details are available yet. “It would depend on how the feds set it up,” he said.

Not every state would jump at the opportunity. Trump’s campaign website says that the school choice money would be given to states as a block grant. It’s unclear if states would then have to use it for a voucher program that could be used at private schools, and whether that could be tricky, given that some states have so-called “Blaine amendments” in their constitutions which prohibit local funds from going to religiously affiliated educational institutions, which includes a lot of private schools.

Back in 2015, Griffith predicted that only a couple of hard-core school choice states, like Arizona, would decide to take advantage of Title I portability. But a lot has changed politically since then, and Griffith says he could imagine a couple dozen states taking a serious look at the proposal, including Florida. Roughly twenty or so other states would almost definitely reject vouchers, including California and Massachusetts. And the first takers would probably be the five states that have education savings accounts for students in special education, he said, particularly if IDEA money could be used for choice.

There are politically less-difficult ways to advance choice, and some of them might come to fruition. Trump or his education allies in Congress could propose a smaller school choice program that’s a lot less ambitious than $20 billion but something more in line with President George W. Bush’s Pell Grants for Kids proposal, which never became a reality. They could also decide to expand the D.C. Opportunity Scholarship Program, which offers vouchers to poor kids in the nation’s capital, and has a mixed track record of success. In fact, 94 percent Education Insiders surveyed by Whiteboard Advisors think the D.C. voucher program has a good shot of becoming a high priority this Congress.

Or Trump and the GOP could go great guns on charter schools, which also receive some federal seed money.