"HERE IS everything you need to know about our plan," House Speaker Paul D. Ryan's (R-Wis.) press office promised in an email offering links to thousands of words of material on the House GOP's new tax-cut proposal. But it did not contain some crucial information that we would like to know and that every lawmaker should have before voting on the package: how the plan would personally benefit President Trump.

Figuring this out would be easy for anyone following the tax-cut debate — if, that is, the public had access to the president's tax returns. But Mr. Trump has broken with decades of bipartisan tradition and refuses to release his tax forms, past or present. So Americans can only guess about the nature and size of his personal stake in the GOP tax plan.

There's reason to guess he might do very well by it.

For example, the House GOP tax plan would eliminate the alternative minimum tax. When summary information from Mr. Trump's 2005 tax return leaked, it showed that the AMT obliged him to pay $31 million more in federal taxes that year. The tax worked in that case just as intended, limiting Mr. Trump's ability to deduct and loophole his taxable income far below what would have been reasonable for someone of his wealth. How many millions would Mr. Trump stand to save just from the AMT repeal?

Similarly, the House GOP proposal would cut rates on "pass-through" income for certain types of privately held companies — the types that Mr. Trump frequently used to conduct his business. Republicans may limit the scale of the break, out of concern that they would otherwise create a new loophole for tax dodgers. But they are still debating how to structure the shift. These details could have large implications for the president's bottom line.

Lawmakers also should not ignore the fact that Mr. Trump's family would almost certainly benefit from eliminating the estate tax, another piece of the GOP plan. Republicans argue that killing the "death tax" is about preserving family farms and other heirloom property. In fact, the tax already applies only to the richest of estates. The Trump family is a better example of the likely beneficiary. Without more information on Mr. Trump's financial situation, it is hard to know by how much.

Refusing to release tax returns is just one of many ways in which Mr. Trump represents a distressing departure from the country's democratic norms. Members of Congress should respond by conducting their own business with the dignity Mr. Trump lacks. A self-respecting legislature would insist on minimal transparency from the executive branch. The House Ways and Means Committee on Monday began several days of formal discussion on the GOP tax-cut plan. The debate should not have even started without some basic clarity about the president's conflicts of interest.