Changing just one number regarding income taxation — as the new administration, some congressional Republicans and probably many more Democrats want to do — could worsen the nation’s fiscal condition and political practices. These damages will occur if Congress retreats from one of its recent achievements.

The 2017 tax reform capped at $10,000 the amount of state and local tax (SALT) payments that can be deducted from federal income taxes. Before then, the unlimited SALT deduction was an unlimited federal subsidy for high-tax, “blue model” state and local governments, most conspicuously in New York, New Jersey, Connecticut and California.

In fiscal 2024, Congress passed no appropriations bill on time or even by the end of the year. In the first three months of fiscal 2025 (October, November, December), the government borrowed $711 billion. Now, with no spending restraint contemplated, the Trump administration and Congress are disagreeing only about how much to raise the SALT cap. This fiscal vandalism is presented as tax “relief.”

Raising the cap to $20,000 for married joint filers would, according to the Committee for a Responsible Federal Budget, mean a 10-year revenue loss of $170 billion, with 94 percent of the “relief” going to households with annual incomes exceeding $200,000 and 0.4 percent to households making less than $100,000. Increasing the cap to $15,000 for individuals and $30,000 for joint filers would make the revenue loss about $450 billion, with an even more regressive distribution of the “relief” that relieves progressive governments of some political disincentives for taxing and spending.

According to the Tax Policy Center, if the SALT cap were removed, the highest-earning 20 percent of households would get 96 percent of the “relief.” Before 2017, the unlimited SALT deduction made it politically easier to implement the blue model of governance: high taxes to fund Democratic regimes that are substantially funded by contributions from public employees unions. To those unions, state and local tax revenue do not trickle down, they flow down like rivers.

House Republicans’ wafer-thin majority gives leverage to any faction determined to exercise it — such as the approximately two dozen Republicans who won last year promising their affluent constituents “relief” this year. One of them, New York Rep. Nicole Malliotakis, says Trump wants to increase the SALT deduction “because he knows that our mayors and governors are crushing taxpayers.”

But the proposed tax “relief” will benefit those mayors and governors, making it politically easier for them to raise taxes. Per the Wall Street Journal’s calculations, for every $1,000 the state and local taxes are raised on a voter in the top tax bracket, he or she gets $370 back from the federal government.

Since enactment of the 2017 tax changes, at least half the states (most under Republican control) have cut their income taxes, thereby further diminishing the benefits their residents get from the SALT deduction. So, raising the SALT cap would further reduce the incentive for conservative policies.

Increasing the SALT cap will increase the deficit, putting downward pressure on defense spending. Nicholas Eberstadt and Patrick Norrick of the American Enterprise Institute note that almost every year for the past two decades, deficits have been larger than defense spending. And defense spending last exceeded social spending during Richard M. Nixon’s first term. Under Jimmy Carter, the cost of social programs was twice the cost of defense. By Bill Clinton’s second term, the ratio was nearly 4-to-1. Today it is 5-to-1.

During candidate Donald Trump’s vote-buying spree last autumn, he promised to end taxation on tips, overtime and Social Security benefits. He cannot end any of those, but an obedient Congress could, with the following estimated (by those Responsible Federal Budget people) revenue losses:

Not taxing tips: at least $100 billion, depending on how imaginatively people connive to get paid in “tips.” Not taxing overtime: between $500 billion and $3 trillion, depending on how aggressively people game the “overtime” category. Not taxing Social Security: around $1.3 trillion. A trillion here, a trillion there, and soon you’re talking about real money.

The American Enterprise Institute’s Kevin Kosar remembers that while campaigning in Kittanning, Pennsylvania, Trump practiced the populism he was preaching. He gave a grocery store customer some cash and a promise: “We’ll do that for you from the White House, all right?”

This is the mentality of much of today’s political class, which Trump reviles and exemplifies. And it is why the $10,000 SALT cap is $10,000 too high.

George Will writes a column for the Washington Post.