A small portion of President Trump’s 2005 tax return is now publicly available. These two pages are more cipher than information. Without the numerous pages of supporting schedules, it is hard to know why Trump was able to claim more than $103 million in losses, offsetting much of the $153 million in income he reported. But one thing is clear: The vast majority of the taxes he and his wife paid that year were a result of the alternative minimum tax. Without it, Trump would have owed just $5 million to the government. The AMT added $31 million. (Trump also owed self-employment taxes, which brought him to $38 million.)

The AMT truly is an alternate tax. When congressional testimony in 1969 revealed that 155 taxpayers with more than $200,000 in income (about $1.3 million in today’s dollars) avoided all income tax liability by using loopholes and shelters, voters were incensed. At that time, making more than $19,000 put you in the top 5 percent of earners, and Congress got more letters that year about these zero-liability taxpayers than about the Vietnam War. So lawmakers passed a series of reforms that culminated in the AMT, designed to prevent wealthy taxpayers from deploying certain deductions to reduce their taxes too much. Even at the time, some questioned this reform: If the deductions were the problem, why not just eliminate them rather than further complicate the tax code?

Today, the AMT is widely panned by Republicans and Democrats; Trump and Bernie Sanders both want it gone. The textbook from which I teach my “Introduction to Income Taxation” course does not mince words: “Everyone agrees it is a terrible provision,” it says. The AMT features different tax rates than the normal income tax structure and denies taxpayers many popular deductions, such as for state and local taxes, personal exemptions, and accelerated depreciation.

If you owe more under the AMT than under the regular tax system, you pay the AMT. The nonpartisan Tax Policy Center estimates that about 4.8 million taxpayers, about 3 percent of those filing returns, will pay it in 2017, raising $35 billion (a bit more than 2 percent of individual income tax revenue).

Detractors note that the provision often targets the upper middle class rather than the truly wealthy. In particular, it hits many upper-middle-class families in high-tax states because it denies the state and local tax deduction and personal exemptions. Almost one-third of taxpayers earning between $200,000 and $500,000 will pay the AMT, and about 62 percent of those earning between $500,000 and $1 million will. Thanks to the deductions it targets (which make a bigger difference for the next-highest brackets), only one-fifth of those earning more than $1 million pay the AMT. For its disproportionate effect on the well-off rather than the fabulously rich, a truly full-throated defense of the AMT is impossible.

Yet for all its flaws and all its enemies, the AMT also solves a real problem. It prevents wealthy and well-advised loophole-seekers from finding ways to evade taxes altogether. It ensures that some people who write off large losses still contribute their share to the national treasury. In 2005, the AMT forced the Trumps, a tremendously rich family, to pay a substantial amount in taxes.

Without seeing his full return, it is impossible to know why Trump’s AMT differed from his regular tax liability or why he faced the AMT when so many other very-high-income earners did not. Possibly it’s because the AMT’s cap on net operating losses limited his ability to offset his 2005 income with losses incurred in previous years. It’s also possible that Trump was affected by the AMT’s limit on depreciation deductions, which normally let taxpayers who buy business investments deduct the cost of those investments as they lose value. The regular tax code allows taxpayers to recoup those costs artificially quickly. The AMT’s depreciation schedules require taxpayers to recover the costs more slowly, resulting in lower depreciation deductions each year.

The upper middle class can survive the burdens of the AMT. For one thing, it’s less of a hassle than it used to be: When taxpayers and their accountants calculated tax liability by hand, the recalculations required by the AMT were understandably difficult. But for many taxpayers today, it’s not hard for TurboTax to crunch these figures. For another thing, a household with income greater than $207,000 is now in the top 5 percent of earners. These Americans don’t need special consideration.

Ultimately, the reason the AMT plagues the upper middle class is that the regular taxes imposed on them are historically low. Proposed income tax cuts would only exacerbate this problem. That’s why repealing the AMT is so difficult. Until we can reform our regular tax code in ways that more equitably raise revenue, it’s also what makes repealing the AMT a bad idea.