Gregory Katsas, a deputy in the White House counsel's office, said Tuesday he advised the Trump administration on many of the issues that may come before the court he's nominated to sit on.

Katsas has spent nine months working as a lawyer for President Donald Trump, who nominated him last month to fill the only vacant spot on the U.S. Court of Appeals for the D.C. Circuit. During a Senate Judiciary hearing on his nomination, senators grilled Katsas about which of Trump's actions he provided legal advice on and his involvement in the ongoing investigation into the Trump campaign led by special counsel Robert Mueller. Katsas in turn gave senators a glimpse into the inner workings of the White House counsel's office.

Before joining the administration, Katsas was a partner at Jones Day, earning roughly $3.9 million last year. But he's no stranger to government service, having worked at the Justice Department from 2001 to 2009. During that time, he held prestigious positions, including chief of the department's Civil Division and as acting associate attorney general. He clerked for both U.S. Supreme Court Justice Clarence Thomas and Judge Edward Becker at the U.S. Court of Appeals for the Third Circuit.

Here are key moments from Katsas' hearing:

Worked on controversial issues: Katsas refused to elaborate on the substance of his advice to the president and the administration, citing executive privilege. He did, however, agree to disclose which presidential actions he's worked on generally. That list included some of Trump's most controversial actions and orders, many of which have spurred litigation.

Katsas said he's worked on the travel ban executive orders, Trump's voter fraud commission, the executive order to protect religious liberty, and the decision to cancel the Deferred Action for Childhood Arrivals program, known as DACA. He also said he advised on the Constitution's emoluments clauses, two ethics clauses that are the subject of several lawsuits against the president, one of which is in the district court in Washington, D.C.

Katsas said that should he be confirmed and any of these issues end up to the D.C. Circuit, he would recuse automatically. An emoluments, travel ban, and voter fraud commission case are each pending in the D.C. district court.

Also worked on Russia investigation, but no recusal promise: Katsas also said he worked on the White House's response to the investigation into possible collusion between the Trump campaign and Russia in the 2016 election. He would not give specifics, citing, in addition to executive privilege, his concern that he would undercut the investigation by providing details. He told Sen. Dianne Feinstein, D-California, that he gave “advice on a few discrete legal questions arising out of the investigation.”

Later in the hearing, Sen. Richard Blumenthal, D-Connecticut, asked Katsas if he would commit to recusal should any of the issues arising from the Mueller investigation make their way before him at the D.C. Circuit. Katsas said he would if the case considered a specific issue he was involved with, but did not say he would recuse from every issue involving Mueller.

“To the extent that … the specific case before me would touch upon anything that I worked on in the White House, yes I would,” he said.

Blumenthal then said that because Katsas declined to disclose which investigation-related issues he advised on, he should recuse from all matters involving it. Katsas repeated he would recuse from any case dealing with matters he was involved with.

“Any matter that I worked on, very easy, open and shut, recuse. And then the recusal statute has two independent grounds for recusal,” Katsas said. “One is if I had knowledge of any material fact in the case. And the other is if my participation would create an appearance of impropriety. I can certainly imagine strong arguments for recusal based on either of those grounds, I just don't want to prejudge it not knowing exactly what the case would be.”

Insight on DOJ flip-flops: Sen. Amy Klobuchar, D-Minnesota, asked Katsas about his opinion on the Justice Department's decision to change its position in several key voting rights cases in Texas and Ohio. Katsas was not involved in either of the cases, but Klobuchar cited an unidentified speech in which Katsas said the Justice Department shouldn't change positions in pending cases without good reason because it could undermine credibility in court.

Klobuchar asked if he still held this belief.

“I think the Justice Department should never lightly change positions,” Katsas said. “What i said was, anytime the Justice Department changes positions in a pending case, they should be very careful to have an explanation that can be publicly articulated and defended on neutral grounds. In my judgement, their change of positions, in the case that I was talking about, seemed very hard to defend.”

Katsas didn't elaborate on which case he was referring to, but the quick moment gives important insight into how he might consider DOJ flip-flops as a judge. The Trump administration already reversed course on many DOJ positions and in pending cases as the president seeks to undo the provisions from the Obama administration.