Grassley’s decision to challenge the Trump administration on congressional oversight is significant not just because he is a Republican, but because as a committee chairmen he would not be affected by the restrictive policy. As he noted in his letter, limiting oversight demands to committee chairmen could shut out many Republicans, although they would have an easier time running their requests through their more senior colleagues. “It obstructs what ought to be the natural flow of information between agencies and the committees, which frustrates the Constitutional function of legislating,” Grassley wrote.

The White House solicited an opinion from the Justice Department’s Office of Legal Counsel, which determined that only chairmen are vested with Congress’s formal power of oversight over the executive branch—a far narrower interpretation of the law than the Obama administration used. “Individual members who have not been authorized to conduct oversight are entitled to no more than ‘the voluntary cooperation of agency officials or private persons,’” the OLC opinion stated.

Democrats assailed the finding as a legal justification for stonewalling; House Minority Leader Nancy Pelosi accused the White House of “an attempted gag order” targeting Democrats who have seen the administration ignore more than 200 requests for documents in the first months of the Trump administration. In a rare departure from the partisan norm, Grassley sided with them. His seven-page letter to the president denounced the OLC’s opinion point by point and accused the office of “a shocking lack of professionalism and objectivity.”

It falsely asserts that only requests from committees or their chairs are ‘constitutionally authorized,’ and relegates requests from non-Chairmen to the position of ‘non-oversight’ inquiries— whatever that means. This is nonsense.

First elected to the Senate in 1980, Grassley has generally been an ally of Trump’s, although he occasionally tries to get the president’s attention through his active and often entertaining Twitter feed. He’s long been a stickler for Congress’s oversight prerogative and frequently jousted with the Obama administration.

Grassley also has first-hand knowledge of how even a single member of the minority party can leverage his position. In 2011, before he became a chairman, he wrote a letter to the Obama administration seeking a response to the account of a whistleblower who claimed that firearms lost in the Justice Department’s Operation Fast and Furious had been used in the murder of a federal border patrol agent named Brian Terry. The letter that the department wrote in reply turned out to be inaccurate, deepening a scandal that prompted the Republican-led House to cite then-Attorney General Eric Holder for contempt of Congress.

Just this week, Grassley testified about the case before the House Oversight Committee and urged fellow lawmakers to pass legislation that would strengthen Congress’s ability to compel documents and information from the executive branch.