This article first appeared at ProPublica. ProPublica is a Pulitzer Prize-winning investigative newsroom. Sign up for The Big Story newsletter to receive stories like this one in your inbox.

As chief U.S. House counsel for four years, Douglas Letter advised then-House Speaker Nancy Pelosi through tense legal standoffs with the Trump administration. He helped shape strategy for the Select Committee to Investigate the January 6th Attack on the United States Capitol, leading to contempt of Congress charges against Trump advisers Steve Bannon and Peter Navarro and subpoenas for five sitting members of Congress.

Now, Letter, a Justice Department attorney for 40 years, has begun a new role as legal counsel for the Brady campaign, defending victims of gun violence and taking on gun laws, such as a local statute in Highland Park, Illinois, that restricts assault weapons like one used in a July 4 parade massacre. Letter said he carries with him lessons learned counseling House Democrats as they faced growing partisan hostilities and concerns for their safety.

In recent interviews, Letter talked about the highlights of his years as House general counsel and his reasons for joining forces with Brady. These interviews have been condensed and edited for clarity.

You led the court fight for release of President Trump’s tax returns and served as counsel on the certification of Joe Biden’s presidential election. Which of the many cases you handled do you consider the most legally significant?

You’re asking me to choose among my children? One is the census case. The Trump administration illegally attempted to add a citizenship question to the 2020 census. And during litigation, lots of evidence was put in the record that they were doing so for a very bad purpose, which was to keep down the count of Hispanic Americans and Asian Americans. So we joined a batch of states and others who were challenging the validity of that. I argued before the Supreme Court, and it’s an interesting opinion. The Supreme Court ruled in our favor, upholding the lower courts, and wrote a fairly narrow opinion but one that is quite meaningful. This was the first time that the Supreme Court had ruled that it did not trust the explanation given by the executive branch. The lower courts had held that the executive branch had acted in bad faith in making it seem like there was a valid justification for doing this. And the evidence showed that that was not true — that the Commerce Department folks who are in charge had asked the Justice Department to basically cook up a rationale. The Supreme Court affirmed and said that the citizenship question had to be stricken. I was very proud of that.

What about Trump v. Mazars, the fight by the House Ways and Means Committee to win the release of six years of President Trump’s personal and business tax returns? That litigation began in 2019 and dragged on until late 2022, just before Congress changed hands.

That’s where we sought private financial information about the president through his accountants and through his bankers. He argued that the House absolutely could not do that. The Supreme Court rejected that argument and said, “That’s absolutely wrong.” The Supreme Court then set a new test that the House had to meet in order to get these materials but did not say we couldn’t get them. Remember, we’re talking about the personal information of the president, and we ended up getting much of the material we wanted. So for us, that was a major victory. The problem was it just took too long.

After Trump left office, you guided legal strategy for the Jan. 6 select committee. What lessons did you learn fighting Trump supporters for documents and testimony?

After the Trump administration ended, the Jan. 6 committee asked the National Archives for the official records of the Trump White House. A federal law passed during Richard Nixon’s time said that those records belong to the people of the United States. President Biden determined that much of the Trump material in the archives was not protected by executive privilege or any other privileges. President Trump disagreed. His argument was completely rejected by the D.C. Circuit Court, a very fast, very thoughtful opinion. And again, the Supreme Court in its shadow docket refused to issue a stay. So all sorts of extremely relevant material was then made available in tranches to the Jan. 6 committee over the next couple of months. That reconfirmed what we already knew, which was that these papers belong to the people of the United States.

You defended Pelosi in a lawsuit brought by three GOP members who were fined for failing to pass through a magnetometer at the House entrance. What did that case — which was thrown out but is now being appealed — reveal about partisan tensions in the House?

Well, It scares me that some members apparently think that it’s okay to bring guns onto the floor of the chamber of the House. If you’re in the House chamber, with all sorts of safety restrictions, you shouldn’t have a major need for self-defense. On more than one occasion, I saw what looked like some members who might go after each other, including during the recent election of Speaker McCarthy. But people intervened, and cooler heads prevailed.

I successfully defended the magnetometer case. But then the new Republican leadership of the House decided to change the policy. That’s their call. We live in a democracy. But Speaker Pelosi, I thought very justifiably, put those measures in place for the protection of other members and staff and security people.

What convinced you to join the Brady campaign?

I was talking to my daughter one morning, and she said she was terrified to send her kids to preschool. Now there are a number of reasons schools can be scary to kids — social reasons — but to be scared because they could get murdered? I’d be stunned if there are many parents in the United States today who don’t have that feeling at one time or another.

And one thing that Brady has pointed out is that Jan. 6 taught us that gun laws work. Some of the crowd were not just people who got carried away by the moment. These were people who had a definite plan set when they came to Washington. And they knew that D.C. had significant gun restrictions. These people cached their weapons in Virginia, across the river. What that meant was that these groups, heavily armed people with very dangerous weaponry, their guns were not at hand because of D.C.’s restrictions. So think about how much worse Jan. 6 — which was horrible — could have been if these people had had their substantial weaponry nearby.

Were you surprised by the catcalls from some Republican members in the House gallery during President Biden’s State of the Union address?

I’m appalled that this is the way the president of the United States would be treated by certain members of Congress as he is speaking. There are rules of decorum, right? I don’t want to sound like some old curmudgeon, you know, “the kids these days.” It seems to me that there are rules of decorum that are to be followed, just as in the military. The Joint Chiefs behave themselves, and, overwhelmingly, the Supreme Court justices behave themselves during the State of the Union. I would expect the members of Congress to do so as well.