The most significant change for schools in the Republican tax reform plan is likely how state and local taxes are handled because of its potential impact on school funding. But why? And where could it have the greatest impact? Buckle up.

Here’s a bit of background: The final bill agreed to by a group of House and Senate lawmakers on Friday imposes a new cap of $10,000 on deductions taxpayers can take for either property and income taxes, or property and sales taxes. That’s much less than what some people, particularly in high-tax states, can deduct now. However, the standard deduction is doubled in the bills, meaning some taxpayers may no longer take state and local tax deductions yet still benefit.

Regardless, state and local governments effectively get a “discount” when they collect revenue. That’s because residents can use the state and local deductions they can currently take to reduce their overall tax burden. Under the final bill, that aforementioned discount could be reduced. That could mean flat or reduced tax revenues, and therefore flat or reduced funding for public schools. Right now, 28 percent of federal taxpayers take state and local deductions.

Now let’s break down key numbers across states. The map below has, for each state, the share of taxpayers claiming state and local tax deductions, the average value of the state and local tax deductions claimed, average per-pupil spending, and the share of that spending from state as well as local sources:

We got the data for the map above from “The Impact of Eliminating the State and Local Tax Deduction.” That’s a report from the Government Finance Officers Association that uses 2015 information from the Internal Revenue Service. We also used U.S. Census Bureau’s “Public Education Finances” report for fiscal 2015 to put the map together.

One general way to think about the map’s numbers is this: As the percentage of people in a state taking state and local tax deductions increases, and as the average size of those deductions increases, it becomes more likely that shrinking those reductions could ultimate reduce school funding. Since the tax plan treats property taxes differently than income and sales taxes, it could create different tax pressures on state or local governments, depending on the current levels of those taxes in each jurisdiction.

But keep in mind, that’s a broad way to look at relevant data. For example, the average state and local tax (or SALT) stats included in the above map represent the mean, not the median. And one student in public school, obviously, does not automatically represent one taxpayer or tax return.

Here are a few other takeaways from the map:

• As you might expect, the deductions are worth the most in relatively high-tax states, although not exclusively. In fact, “Six states—California, New York, New Jersey, Illinois, Texas, and Pennsylvania—claim more than half of the value of the deduction,” said the Tax Foundation, a think tank, in a report from earlier this year.
• However, where taxpayers do take them, the deductions also are relatively high in some states controlled by Republicans, including Iowa and Wisconsin.
• Utah and Georgia are two GOP-run states where a relatively high share of individuals take the deductions.
• There’s no state where a majority of taxpayers take the state and local deductions. Maryland is the state where the largest share of taxpayers claim those deductions, at 46 percent, according to the Government Finance Officers Association.

“The question is: How does it change your incentives to support spending on schools?” said Nora Gordon, an associate professor at Georgetown University who studies school finance.

Before lawmakers agreed to the final bill, Americans for Taxpayers Fairness ran a multistate analysis of the impact of allowing taxpayers to only deduct up to $10,000 in combined property and income taxes, which is one (but we must stress not the only) option taxpayers would have under the legislation. Here’s what the group found for California:

And here’s the chart for another big state, Florida, but one with a very different tax structure:

But what about at an even more localized level, in counties? Let’s take Montgomery County, Md., where Education Week is located, as an example—it is an extreme example, given that it is the 19th wealthiest county in the U.S., according to the Census Bureau. It has a median household income of $98,314, has relatively high local property taxes, and is located in a relatively high-tax state. The Census Bureau estimated its population to be just over a million in 2016. Here’s how the National Association of Counties, which opposes eliminating the state and local deductions, breaks down the value of those deductions to Montgomery County:

That tiny print at the bottom points out that the counties association defines “middle income” as people making less than $200,000 annually in adjusted gross income, a calculation that’s certainly debatable. But the point remains that the SALT deduction was claimed by hundreds of thousands of residents, and that the value of those deductions was relatively high, on both an aggregate and per-capita basis.

Here’s another extreme example: McDowell County, W. Va. It is one of the poorest counties in the U.S.—based on a median household income of $22,252— and which (perhaps not coincidentally) has previously asked the state to take over its K-12 system, and where an education coalition (including the American Federation of Teachers) has also stepped in to help schools. The Census Bureau estimated its population to be approximately 19,100 people.

In many smaller counties with a smaller tax base, the state and local deductions mean significantly less to fewer people, especially in the aggregate.

Just for kicks, let’s look at a county with a median household income about half-way between that in Montgomery and McDowell—Orange County, Va., which has an estimated population of about 35,500.

You also have noticed that while getting rid of those deductions has a bigger overall dollar impact in Montgomery County, there was a smaller share of middle-income households claiming the deductions than in McDowell County and Orange County. The Tax Foundation has a simple explanation for this: “The lion’s share of state and local tax deductions are claimed by upper-income earners.”

“State and local tax deduction, in addition to being a guiding principle of state and local tax autonomy, is essential to local governments funding public services,” said Jack Peterson, an associate legislative director at the National Association of Counties. “If deductability goes away, state and local governments are much more likely to see pressure ... on their local tax rates.”

Here’s a general point of contention: If the tax reform succeeds in growing the economy, as GOP congressional leaders have stressed, that creates a bigger tax base for all levels of government and more funding available for public services. But Democrats and many school advocacy groups say reducing the deductions is still unfair, and could reduce the services provided to students.

“What it’s officially doing is putting a tax on a tax,” said Thomas Gentzel, the executive director and CEO of the National School Boards Association.

Finally, getting rid of tax deductions is not identical to a direct funding cut. It would still be up to state and local governments to decide how much to tax their residents, and how much of the resulting tax revenue to spend on schools. Some jurisdictions might decide that even without the “discount” we discussed above, schools are crucial enough that their funding shouldn’t be impacted by changes to federal taxes. Peterson acknowledged this, saying, “There is still an element of local decisionmaking” even without reduced state and local deductions. But with them in their current form, he said, that power is greatly enhanced.

Votes on this bill in the House and Senate should take place this week. If it passes both chambers, it goes to President Donald Trump for his signature.