Gregory Katsas testifies before the Senate Judiciary Committee at his confirmation hearing on his nomination to be assistant attorney general in 2008. (Dennis Cook/AP)

President Trump tapped one of his White House lawyers Thursday for an open seat on the U.S. Court of Appeals for the District of Columbia Circuit, which often rules on important political cases involving executive power and government regulations.

Gregory G. Katsas, whose nomination requires confirmation by the Senate, has served as Trump’s deputy legal counsel since March, held senior posts at the Justice Department during the George W. Bush administration and was a law clerk for Supreme Court Justice Clarence Thomas.

If confirmed, Katsas would join a bench referred to as the nation’s second-highest court — because of its decisions on ­separation-of-powers issues and because it has been something of a pipeline to the Supreme Court, with three current justices having previously served on the D.C. Circuit.

Pending now before the court is President Barack Obama’s signature effort to combat global warming, the constitutionality of a key consumer watchdog agency and gun-control laws in the nation’s capital.

Katsas, 53, is considered a conventional conservative and would replace another conservative, retired judge Janice Rogers Brown, whose opinions were often infused with a libertarian streak.

Former colleagues describe Katsas as quirky and unassuming despite his intellect and legal credentials. He graduated from Princeton University, was an executive editor of the Harvard Law Review and was a clerk for Thomas at the D.C. Circuit in addition to the high court. Katsas has argued more than 75 appeals, according to the White House announcement, including the challenge to the Affordable Care Act at the Supreme Court.

Stephen Vladeck, a University of Texas law professor, said Katsas’s work at the Justice Department during the Bush administration and at the White House as Trump’s legal adviser suggests that he would be sympathetic to strong views of presidential power.

“There’s no question that he ticks all the most important ­boxes for government experience, but that experience is very pro-executive-branch,” Vladeck said.

In the aftermath of the Sept. 11, 2001, terrorist attacks, Katsas was deeply involved in high-level, sensitive policy decisions involving national security and civil rights.

“Greg had to make all the hard calls,” said lawyer Robert Loeb, who worked with Katsas at the Justice Department on cases involving foreign detainees at the military prison at Guantanamo Bay, Cuba, and other matters.

“He was not a knee-jerk conservative. He was always thoughtful and balanced in his approach to the tough decisions,” Loeb said. “He always dug into the issues himself and sometimes ended up taking positions that did not make everyone in the White House or Defense Department happy.”

During the Obama administration, Republicans tried to block the president’s nominees to the D.C. Circuit, saying the court’s workload did not require Obama to fill all 11 seats. Obama eventually put four judges on the court.

In contrast, on Thursday, after Trump’s announcement, the conservative Judicial Crisis Network almost simultaneously unveiled a new website backing Katsas. The site details Katsas’s biography and includes more than a dozen statements of support from partners at Washington law firms, former U.S. attorney general Michael B. Mukasey and former White House counsel C. Boyden Gray.

Carrie Severino of the Judicial Crisis Network said Katsas “has shown he will honor the Constitution and fairly apply the law.” She described his nomination as “yet another example of President Trump’s commitment to nominating qualified, experienced constitutionalists to serve on the courts.”

Katsas lives in Falls Church, Va., with his wife and two children.

Correction: An earlier version of this report referred to the Judicial Crisis Network as the Judicial Action Network in one reference.