The congressional Democrat who has the power to get President Donald Trump’s tax returns has made his move.

On Wednesday, Rep. Richard Neal (D-MA), who chairs the House Ways and Means Committee, sent a letter to the IRS commissioner demanding Trump’s federal income tax returns for the past six tax years.

Neal’s letter asks for the 2013-2018 federal income tax returns for the president himself and for eight entities connected to Trump (including the Donald J. Trump Revocable Trust and seven Trump business entities). He also demands information about any audits connected to these returns, and administrative files for them.

“Congress, as a co-equal branch of government, has a duty to conduct oversight of departments and officials,” Neal said in a statement. He framed his request not as about scrutinizing Trump’s own history, but about evaluating an IRS policy to audit all presidents’ tax returns.

Neal wants the returns by April 10 — Wednesday of next week. But this is likely only the start of a lengthy legal battle.

Trump’s tax returns: the background

For decades, every major party presidential nominee publicly released their tax returns, in a gesture that wasn’t legally required but was a nod to transparency. Such disclosures could reveal more about how politicians have made their money, whom they might owe money to, or whether they were in fact appropriately paying their taxes.

Very quickly, Trump — a wealthy businessman with a complex, opaque financial situation — was asked whether he would follow these past candidates’ example. He said he would. In February 2015, while Trump was considering running, he told conservative radio host Hugh Hewitt that if he ran, “I would release tax returns.” Then, after getting in the race, Trump told ABC in August 2015 “at some point I’ll release it.”

As the first primaries and caucuses approached the following January, Trump told NBC again that such a release was coming soon. “We’re working on that now. I have big returns, as you know, and I have everything all approved and very beautiful and we’ll be working that over in the next period of time.”

But in February 2016, Trump’s line suddenly changed. The problem, he said, was that he was currently being audited by the IRS. Because of that, he said, he wouldn’t release his returns until whenever the audit was finished. (Sometimes Trump would go further and say that the audit meant he “can’t” release his returns, which is false.)

Tax experts throughout the media agree that no sane person would give their tax returns during an audit. After the audit, no problem! — Donald J. Trump (@realDonaldTrump) February 27, 2016

Many immediately suspected the audit excuse was somewhat less than sincere. Perhaps the best evidence this is the case is that more than three years have passed since then, and Trump still hasn’t released his returns.

Indeed, after he won the presidency, Trump shifted to arguing that he shouldn’t have to release his returns because he, uh, won the presidency.

I did what was an almost an impossible thing to do for a Republican-easily won the Electoral College! Now Tax Returns are brought up again? — Donald J. Trump (@realDonaldTrump) April 16, 2017

Last year, the New York Times wrote a lengthy report on Trump’s history of participating in “suspect tax schemes,” including “instances of outright fraud.” But the paper relied mainly on returns from the 1990s — those from more recent years remained elusive. And congressional Republicans — who had control of the House and Senate for Trump’s first two years in office — weren’t particularly eager to learn more.

An obscure law lets the Ways and Means committee chair request anyone’s tax returns

But the Democratic takeover of the House of Representatives in last year’s midterms finally provided an opportunity for Trump’s returns to be brought to light.

A law passed in 1924 gives the chair of the House Ways and Means Committee the power to request any individual’s tax returns from the Treasury Department, for review in closed session. Here’s the key part of the Internal Revenue Code:

Starting this January, that chair was Rep. Richard Neal. Democrats had campaigned on — among other things — holding Trump and his administration accountable on a number of issues, but have been somewhat slow to confront the administration. This week, however, Neal finally invoked his authority to demand six years’ worth of returns from Trump, Trump’s trust, and “seven other core Trump business entities that control scores of other Trump operations” (per the New York Times).

Don’t expect the returns to be handed over quickly — Neal himself has said he expects a court battle over this matter. And even if the Treasury Department does provide Neal with the returns, there’s another catch — they’re supposed to be reviewed only in “closed executive session,” so it’s unclear how they’d be able to be made public.

But Neal’s request is the first major step in this process. You can read his full letter below, or at this link.