President Trump is tapping a young district judge and former clerk to now-Supreme Court Justice Brett M. Kavanaugh to fill a vacancy on the powerful U.S. Court of Appeals for the District of Columbia Circuit.

The nomination of Justin Walker, 37, is almost certain to trigger a vigorous battle over his qualifications to serve on the nation’s second-most influential court, which is often at the apex of key cases involving executive power and federal regulations. The American Bar Association had deemed Walker not qualified for his current post as judge for the Western District of Kentucky, where he has served for just over five months.

But Walker’s nomination also underscores the influence of Senate Majority Leader Mitch McConnell (R-Ky.) on the conservative transformation of the federal judiciary under Trump, to whom McConnell has actively promoted Walker behind the scenes.

“I think you cannot credibly argue that Justin Walker is not a judicial all-star,” McConnell said in an interview with The Washington Post in advance of the announcement Friday, ticking off his academic and legal credentials.

Chief among them are Walker’s clerkships with former Supreme Court Justice Anthony M. Kennedy and Kavanaugh during his tenure on the D.C. Circuit. Both Kennedy and Kavanaugh privately recommended Walker for the D.C. Circuit vacancy in conversations with Trump, according to a person familiar with the calls who spoke on the condition of anonymity to speak freely.

Even more active was McConnell. He knows Walker’s grandfather and first met Walker when he interviewed the future majority leader for an article for his high school newspaper. McConnell had previously recommended Walker for the Western District seat and in January, accompanied the judge to the Oval Office to meet the president, although that conversation would ultimately veer to Trump’s then-looming impeachment trial.

Trump, McConnell recalled, came away impressed with Walker during the 15-minute conversation and connected well with him. Walker is the youngest nominee to the D.C. Circuit since 1983, and the first from outside Washington since 2005.

“I thought it might be a good idea to go outside the Beltway” for the D.C. vacancy, McConnell said. “There are plenty of D.C. lawyers that salivate over this job.”

The close ties among McConnell, Kavanaugh and Walker were highlighted last month when the majority leader returned home to Kentucky to attend Walker’s investiture in Louisville as Congress wrestled with a second aid package to combat the effects of the coronavirus pandemic. Both donning the traditional black robe of a judge, Kavanaugh swore in his former clerk, who fervently defended him in the media during his contentious 2018 confirmation fight.

Walker conducted 162 media interviews — including 35 appearances on Fox News, the president’s favored network — defending Kavanaugh and his character, according to the Leadership Conference on Civil and Human Rights, which opposed Walker’s nomination to the district court seat last year, citing his ABA rating and “ideological defense” of Kavanaugh.

Walker would fill a vacancy being created by the retirement of Judge Thomas B. Griffith, a George W. Bush appointee who indicated early last month that he will retire in September. Trump has had two picks confirmed to the influential appellate court: Gregory G. Katsas, the president’s former deputy legal counsel, and Neomi Rao, who was Trump’s regulatory czar before her judicial nomination.

Demand Justice, a liberal advocacy group focused on the judiciary, has raised questions about the circumstances surrounding Griffith’s decision to leave the bench because it came shortly before reports that McConnell was quietly reaching out to judges on their plans to retire.

“The nomination of a Mitch McConnell crony, who has been rated unqualified by his peers, to the second highest court in the country is beyond suspicious,” said the group’s chief counsel, Christopher Kang, who worked on judicial nominations in the Obama White House. “We need an immediate investigation into whether McConnell manufactured this vacancy by unethically pressuring Judge Thomas Griffith to retire now.”

The D.C. Circuit has ruled on an array of cases directly involving Trump and his agenda — such as access to his financial and tax records, congressional subpoenas and his administration’s policies. Four of the current Supreme Court justices previously served on the D.C. Circuit — Kavanaugh, as well as Chief Justice John G. Roberts Jr., and Justices Clarence Thomas and Ruth Bader Ginsburg.

A graduate of Duke University and Harvard Law School, Walker has served as law professor at the University of Louisville since 2015. He is a member of the Federalist Society, a powerful network of conservative lawyers, which has held an outsized influence on Trump’s judiciary strategy and his picks.

The biggest mark on his record has been his “not qualified” rating from the nonpartisan American Bar Association, which told the Senate that Walker did not have the requisite courtroom experience to serve as a judge.

“Mr. Walker is less than 10 years out of law school, has never tried a case, has never served as co-counsel, and it’s not clear how much of his 10 years has even been spent practicing law,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said in a floor speech opposing his nomination.

Republicans have frequently dismissed the ABA’s ratings of Trump nominees. Walker was confirmed on a party-line, 50-to-41 vote in October.

Walker’s nomination to the D.C. Circuit is already attracting support from the constellation of influential conservative figures and groups who have engineered a successful pipeline of judicial confirmations under Trump.

In an email to The Post, former White House counsel Donald McGahn, a pivotal architect of the Trump administration’s judiciary efforts earlier in the presidency, called Walker an “outstanding choice.”

“He has exceptional academic credentials, clerked at the U.S. Supreme Court, and has focused his legal scholarship on precisely the sort of issues that come before the D.C. Circuit,” McGahn said. “Judge Walker fits the mold of the president’s federal appellate court nominees, I applaud his selection, and wish him a speedy confirmation.”

Carrie Severino, president of the deep-pocketed Judicial Crisis Network, said the group is “prepared to do whatever necessary” to ensure Walker’s confirmation. And the Article III Project, led by Mike Davis, who was the chief nominations counsel for the Senate Judiciary Committee during the Kavanaugh confirmation fight, will lead the outside effort to get Walker confirmed.

“Judge Walker has had an impressive legal career, during which he has aligned himself with originalism, textualism, and the important separation of powers principles that make our country great,” said Leonard Leo, who has worked closely with both the White House and the Senate on judicial nominees

For three years, McConnell has moved at a rapid tempo to fill the judicial openings inherited by Trump after the GOP Senate leader refused to act on dozens of President Barack Obama’s nominees, most notably Supreme Court nominee Merrick Garland, who has served on the D.C. Circuit since 1997.

McConnell has vowed to leave no vacancy behind as the president faces reelection this November, and Senate Republicans face the risk of losing control of the chamber.

McConnell has paid particular attention to the nation’s appellate courts, one step below the Supreme Court where the vast majority of cases stop. Trump has 51 circuit court appointees, which translates to 1 out of every 4 appellate judges. McConnell says the circumstances are different now because both the Senate and the White House are of the same party, which was not the case four years ago.

In the interview with The Post this week, McConnell said the Senate Republican majority’s commitment to confirming judges is unchanged despite a congressional calendar made uncertain by the coronavirus pandemic, preventing senators from holding confirmation hearings and casting votes. The Senate, under McConnell, has installed 193 of Trump’s picks for the bench, including two Supreme Court justices, 138 district court judges and two judges for the U.S. Court of International Trade.

“This Congress goes on until Dec. 31, and we intend to confirm all of the judges that are sent up to us this year,” McConnell said. He also reiterated his promise that he would fill a Supreme Court vacancy this year, should there be one, a reversal from his argument in 2016 that a seat should remain open until the voters decide in the presidential election.