President Donald Trump s legal battle with Congress over the legislatures authority to subpoena his financial documents hinges on a small handful of decades-old Supreme Court precedents.

But as Democrats and two judges whove ruled on the dispute have noted, the cases Trumps lawyers have decided to rest his legal defense on actually affirm congressional power.

The earliest Supreme Court ruling on Congress investigative authority is Kilbourn v. Thompson, an 1880 case that established that Congress can only pursue investigations as an extension of its power to craft legislation. At the time, this ruling limited Congress from pursuing an investigation into a private individual, even if it could lead to future legislation. This is why Trumps lawyers and those at the Treasury Department repeatedly claim that Congress lacks a legitimate legislative purpose to enforce subpoenas.

But subsequent decisions, even those cited by Trumps lawyers, overruled Kilbourns limitations on congressional investigations essentially by ruling that just about anything can be a legislative purpose.

In 1927, the Supreme Court upheld Congress investigatory power and its right to hold uncooperative witnesses in contempt in the unanimous McGrain v. Daugherty case. This case, heavily cited by House Democrats in their court briefs and arising out of the Teapot Dome bribery scandal , affirmed Kilbourns test that Congress needs a legislative purpose for its investigations, but read that legislative purpose as expansive.

Congressional investigators do not need to introduce or point to legislation to request relevant information. Requests for documents or testimony need only concern, a subject on which legislation could be had which would be materially aided by the information which the investigation was calculated to elicit, according to the court.