The new tax bill, “Tax Reform 2.0,” is here, and it makes permanent the $10,000 cap on the state and local tax deduction (SALT) created by the Tax Cut and Jobs Act (TCJA) in December 2017.

Meanwhile, Republican politicians from districts where high percentages of taxpayers will be affected by the cap are wary of making the cap permanent. A deeper dive into theories of taxpayer psychology and tax policy indicates these politicians are right to be concerned.

First, consider how the cap will shrink refunds or increase tax bills for millions of taxpayers.

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Early next year, Jane fills out her tax return using her preferred commercial tax prep software. She enters her property tax information, expecting, as in prior years, that her federal tax liability will drop considerably. But she surpassed the SALT cap of $10,000 when she previously entered her $12,000 in-state income taxes paid.

Under prior law, and when added to her $8,000 in property taxes, Jane would have received a $20,000 federal tax deduction. But the new law caps her deduction at $10,000, and so she loses the value of the additional $10,000 deduction.

How much does she lose? If she’s in the 24-percent tax bracket, Jane is worse off by $2,400, either resulting in a smaller refund or more tax.

And it gets worse for higher-income taxpayers. If Jane had a total of $110,000 in SALT paid rather than $20,000, she would lose the value of the additional $100,000 deduction. Since Jane would likely be in the top tax bracket of 37 percent, she’d owe $37,000 more in taxes.

Beyond the sting of owing more in tax, Jane also may feel that she is being punished for doing the right thing: opting to pay more in state and local taxes in exchange for better state and local public goods.

Research indicates that dismay at this tax change might be quite politically salient to the taxpayer when making voting decisions. Compare the large — and explicit — jump in tax liability described above to an increase in withholding taxes from periodic paychecks.

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Moreover, millions of taxpayers are likely to react negatively to their higher-than-expected tax liability even if some of those same taxpayers pay lower taxes in the aggregate due to other changes in the law.

Indeed, the SALT cap is arguably already impacting property owners in jurisdictions whether they itemize or not because itemizing home buyers understand that their future property taxes will no longer be deductible over the cap and are accounting for that change in their home-buying budgets.

Homes are typically a taxpayers’ largest asset and voters can be aggressive in voting to protect the value of that asset.

Retaining the SALT cap is also fraught with political peril because taxpayers are sensitive to the reality and perception of procedural and substantive fairness. It is unlikely that the partisan, rushed, secretive and demonstrably flawed processes that produced TCJA and now Tax Reform 2.0 are going to be perceived as fair.

As for substantive fairness, Republicans have claimed repeatedly that most taxpayers will receive lower tax bills. But it is hard to explain why two-earner families in a handful of states should not get a tax cut on account of the SALT cap, to say nothing of the unequal political valence of the jurisdictions targeted by Congress with a tax increase.

It is possible for voters to be convinced by more abstract tax policy arguments. And politicians are to be commended when they pursue the correct policy and endeavor to persuade their constituents. But the policy arguments for the SALT cap are feeble.

One argument is that the cap is progressive in that wealthier taxpayers are affected by it. But the cap was embedded in a very regressive bill. A tax change that only subjects a small sliver of wealthier taxpayers — though not the wealthiest — to higher taxes is not fair.

Another unpersuasive argument is that the SALT cap corrects for the fact that low-tax states were subsidizing high-tax states through the deduction. For starters, this kind of inter-state accounting is corrosive to our polity.

Worse, it leaves out a key piece of information. Specifically, the states that have the most taxpayers affected by the cap are among the wealthiest states and thus are net “givers” to our common government. Capping the SALT deduction makes these states’ relative contribution even higher.

Another argument is that a full deduction for state and local taxes might not be appropriate as a matter of tax principle. Fair enough, but it’s incongruous in the context of a tax bill loaded with tax policy blunders and deviations from income tax principles, most notably new code section 199A, the so-called passthrough deduction, which is a bacchanal of unprincipled and regressive income tax policy.

Phase 2.0 will make this blunder permanent, so the claim that the SALT cap is a principled change rings quite hollow.

In any event, a more typical approach to situations where it is uncertain how much of a deduction is proper would have involved using a percentage cap, like 50 percent. Limiting the SALT deduction in such a way would have been unpopular as well, but it would have been principled. Other principled options exist.

Perhaps the biggest political danger posed by the SALT cap is that it could create millions of apostles carrying a simple message: Ultimately, very few taxpayers will receive their promised tax cut from Republican tax changes.

This is because borrowing trillions of dollars (with interest) to reduce taxes does not a tax cut make. While taxpayers at the very top of the income ladder will realize huge tax savings both now and in the future, the rest of us will ultimately face higher tax bills and/or fewer vital services.

The millions of taxpayers paying more in taxes next year due to the SALT cap are just the harbingers.

Darien Shanske and Dennis Ventry are professors at the University of California Davis School of Law. Shanske's areas of academic interest include taxation, particularly state and local taxation, local government law, public finance and political theory. Ventry is an expert in tax policy, tax practice and tax filing and administration.