The fight for Congress to obtain President Trump’s tax returns centers on one question: Is Congress requesting them for a legislative reason?

No one disputes that Congress has a right to request them. A 1924 law gives House Ways and Means Committee Chairman Richard E. Neal (D-Mass.) the ability to request any taxpayer information from the Internal Revenue Service. (Here’s the backstory on the bribery scandal that led to that law.) It’s a powerful law that allows the chair of Congress’s main tax-writing committee to get a person’s tax returns as well as underlying files about that person’s taxes, such as the W-2 form that details how much someone made in a year. If there’s something Trump has been hiding from the public in his tax returns for the past six years, this law ensures that the committee should see it. (And potentially make it public.)

But members of Congress can’t request someone’s tax returns just because they want to. That would be ripe for abuse. They must have a reason to see the taxes that directly relates to their job as part of a lawmaking body.

So what legislation is Congress going to introduce, or consider introducing, based on Trump’s tax returns? That’s where the Trump administration thinks it has a case to make.

“I have determined that the Committee’s request lacks a legitimate legislative purpose,” Treasury Secretary Steven Mnuchin wrote in a letter to Congress on Monday, when telling lawmakers that he won’t be sharing the president’s tax returns.

Neal has argued that his legitimate legislative reason is this: The IRS has a policy that every year it reviews the tax returns of the president and vice president to make sure they’re not doing anything fishy and to instill public confidence that they aren’t above the law.

But Congress has no idea how thorough the IRS is with this. Does it do a full audit? A cursory check looking only for red flags? A Democratic committee aide pointed out that the IRS has appeared to go easy on a president before. Richard Nixon once received a letter from the agency saying that his taxes were a-okay, only for Congress to find out that he owed $500,000 in interest and back taxes.

This is all IRS policy, not a law written by Congress. Trump is the first president in decades not to share his taxes with the public, so Democrats argue that this is a good time to consider giving Congress more oversight into how the IRS reviews a president’s taxes. To do that, it needs to request this president’s returns.

We don’t know exactly why the administration thinks this isn’t a “legitimate legislative purpose.” Mnuchin wrote in his letter to Congress that the Justice Department, which he consulted for this decision, is going to share its reasoning soon.

But we can deduce that the argument has something to do with the timing. Democrats won control of the House and immediately opened half a dozen investigations of Trump and his presidency and business practices. Did Democrats decide they wanted to get Trump’s tax returns and craft a legislative reason around it?

Trump’s Republican supporters argue that that scenario seems plausible, if not likely. “Big difference [between] legitimate congressional oversight & using the IRS as a political weapon,” tweeted Sen. Charles E. Grassley (Iowa), chairman of the Senate Finance Committee.

Democrats on this panel counter that they’re pretty sure they’re on firm legal ground. The 1924 law says the IRS “shall furnish” the tax information to Congress.

And Neal is known as someone who pins his requests on policy rather than politics. He has been expecting a legal fight, so he was very careful not to overstep his requests. He asked for only six years of Trump’s tax returns rather than, say, 10 or 20 years, because he thinks that number abides by the 1924 law. He asked for returns from only some of Trump’s businesses, such as his golf club in New Jersey. (Democrats say they requested enough to paint a picture of the “core of the president’s business empire.”)

Like almost every other piece of information House Democrats have requested from Trump, they’re going to have to fight the president in court to get it. And the thrust of this debate will be whether they have a legitimate legislative reason to request the tax returns. To be continued.