House Democrats are finally taking the Trump administration to court over the president’s tax returns, but don’t expect a resolution soon.

The House Ways and Means Committee, which is chaired by Rep. Richard Neal (D-MA) and is one of the few congressional committees with the authority to request tax information from the IRS, on Tuesday filed a lawsuit in federal court to enforce a subpoena for President Donald Trump’s tax returns. The suit names the Treasury Department, the IRS, Treasury Secretary Steven Mnuchin, and IRS Commissioner Charles Rettig as defendants.

This is the latest development in a fight over Trump’s tax information, which the president has insisted on keeping under wraps, despite decades of US presidents releasing their tax returns voluntarily.

In April, House Democrats asked the IRS to hand over six years of Trump’s tax returns. They involved Section 6103 of the Internal Revenue Code, a statute dating back to 1924, that authorizes the Ways and Means Committee to request from the Treasury Department the tax return information — personal business — of any taxpayer. (I have a full explainer on the statute and how Democrats could use it here.) Treasury, theoretically, has to comply. But it didn’t.

Mnuchin missed two deadlines to hand over the returns and eventually officially denied Democrats’ request. That prompted the subpoena from Neal, but the Trump administration still held firm. Mnuchin’s argument: Democrats lack a “legitimate legislative purpose” for requesting the returns and therefore the department can’t disclose Trump’s returns. “For the same reasons, we are unable to provide the requested information in response to the Committee’s subpoena,” Mnuchin wrote in a May 17 letter to Neal. In June, the Justice Department issued a memo backing the secretary up.

In Tuesday’s lawsuit, Democrats argue that Section 6103’s language that the Treasury Department “shall furnish” the committee with “any” tax return information it requests is mandatory, not optional. “In refusing to comply with the statute, Defendants have mounted an extraordinary attack on the authority of Congress to obtain information needed to conduct oversight of Treasury, the IRS, and the tax laws on behalf of the American people who participate in the Nation’s voluntary tax system,” the lawsuit reads.

Democrats are investigating the IRS’s administration of “various tax laws and policies” related to presidential tax returns and tax law compliance, the lawsuit continues, including whether the IRS’s policy of auditing sitting presidents annually is actually happening.

Rep. Bill Pascrell (D-NJ) has been making the case for pursuing Trump’s tax returns under Section 6103 basically since Trump was inaugurated. In an emailed statement on Tuesday, he said that litigation is “always a last resort,” but this is what has to be done.

“We are here today because Donald Trump and his enablers have sneered at our laws and avoided the thinnest accountability for their corruption. The Ways and Means Committee must file this action to stand for rule of law. Despite Republicans’ efforts to cover-up, Americans cry out for oversight and voted in a new House to enforce that demand in November,” he said.

As the Wall Street Journal notes, this court battle is going to take a long time — months or even years. It seems likely it could wind up in the Supreme Court.

Neal has taken a cautious approach to getting Trump’s tax returns and has spent months trying to lay out a legal case with the best chance of prevailing in court. Some Democrats have criticized him for taking it slow. HuffPost on Monday ran a story on frustrations about the pace of the tax returns request, the day before Neal made his move.

The Treasury Department did not immediately return a request for comment.