Democrats on Friday issued subpoenas for President Donald Trump’s tax returns after the Trump administration said it would not comply with a formal legal request for the documents this week.

“While I do not take this step lightly, I believe this action gives us the best opportunity to succeed and obtain the requested material,” said Rep. Richard Neal (D-Mass.), chairman of Ways and Means, the chief tax-writing committee of the U.S. House.

Neal’s announcement marks the beginning of a legal battle that could drag on for months or years, though nobody knows for sure how long it could take because the situation is unprecedented.

The tax returns represent just one of many Democratic requests that the Trump administration has blocked while assuming an unprecedented posture of stonewalling. Federal judges have generally upheld the congressional power to investigate the executive branch, though court cases can outlast sessions of Congress.

The tax fight is different from the other oversight battles, however, because a long-standing federal law explicitly gives Congress access to anyone’s private tax information. The chairs of tax committees have routinely used the statute to help with drafting legislation and overseeing the executive branch. Requests usually go to the commissioner of the Internal Revenue Service, and experts are unaware of any previous request being denied.

But after Neal asked for six years’ worth of Trump’s tax returns last month, Treasury Secretary Steve Mnuchin swooped in to say the request was probably unconstitutional. Mnuchin formally refused to comply with the law on Monday.