PRESIDENT TRUMP probably was untruthful when he insisted last week that his tax plan "is not good for me." We can't be certain, because Mr. Trump is asking us to take his word on the matter; he won't release his tax returns. He is the first president in modern times, in fact, to so refuse. Before last week, questions centered on what his tax records might show about his connections, if any, to Russia. Now Mr. Trump has himself posed another crucial question: whether he is pushing tax cuts that could profit him and his family to the tune of millions of dollars.

This is no idle speculation. Though vague on crucial aspects, such as how to pay for it, Mr. Trump's tax reform outline contains several specific proposals that would help the wealthy. The Urban- ­Brookings Tax Policy Center released Friday an analysis showing that, by 2027, the GOP tax plan would provide massive benefits to upper-income taxpayers even as it smacked large sections of the middle class with a net tax hike.

Mr. Trump in particular might benefit from the elimination of the alternative minimum tax (AMT), which prevents taxpayers from using deductions and loopholes to avoid paying at least a minimum amount to the federal government. There are some reasons to criticize the AMT; it tends to hit those with moderately high incomes harder than the super-rich, for example. But there is at least one prominent example of the AMT working as designed — on Mr. Trump's 2005 tax bill. Mr. Trump's return from that year, and that year alone, was leaked in March, and it showed that the AMT obliged him to pay $31.3 million more in taxes that he otherwise would have.

Then there is Mr. Trump's idea to eliminate the estate tax. Republicans pretend killing the "death tax" is about allowing family farmers to pass their land to their children without government interference. In fact, the tax affects only the wealthiest of estates — those worth $5.49 million or more, twice that for a married couple. The Tax Policy Center estimated that only about 80 small farms and businesses would pay estate tax this year. No one fortunate enough to qualify to pay the estate tax requires sympathy — let alone a budget-busting change in tax law. But its elimination could be a great favor for Mr. Trump's heirs.

A drastic change in the taxation of business "pass-through" income also could greatly benefit businesses such as the Trump Organization. Again, it is impossible to know for sure, given the secrecy shrouding Mr. Trump's finances.

Mr. Trump began his presidential campaign promising to release his tax returns, as every major-party candidate has done for decades. But first, he said, he had to wait for the Internal Revenue Service to finish auditing them — though an audit, even if one were going on, would not legally prevent him from releasing his returns and certainly would not keep him from releasing prior years' returns. Then he dismissed continuing questions by insisting that only reporters care — though polls show that isn't true, either.

So, on one side, there are specific elements of the president's tax plan that would benefit the wealthy at the expense of the nation's fiscal health. On the other side is an empty space where Mr. Trump's tax returns should be. A Congress with integrity would agree to consider his tax proposal only after he published his tax returns.