David Barron testifies before the Senate Judicary Committee during his nomination hearing on Nov. 20. (Chip Somodevilla/Getty Images)

The White House pledged Tuesday to give lawmakers expanded access to memos on the legality of killing American citizens in drone strikes, a concession aimed at heading off Senate opposition to a judicial nominee involved in drafting those secret documents.

The move was designed to salvage the nomination of David J. Barron to the U.S. Court of Appeals for the 1st Circuit and address growing frustration among lawmakers over secrecy surrounding the administration’s counterterrorism operations a year after President Obama vowed greater transparency.

Barron, who previously worked in the Office of Legal Counsel at the Justice Department, was a principal author of at least one memo that served as the legal foundation for Obama’s decision to order a 2011 drone strike that killed Anwar al-Awlaki, a U.S. citizen who had become a senior al-Qaeda operative in Yemen.

White House spokesman Eric Schultz said the administration would make “available in a classified setting a copy of the Awlaki opinion to any senator who wishes to review it prior to Barron’s confirmation vote.” The documents previously had been made available only to members on certain committees.

The statement came after several senators threatened to oppose Barron’s nomination unless Obama agreed to give members full access to the Awlaki memo and other drone-related documents the White House has fought to keep secret.

The White House gesture failed to sway some lawmakers, including Sen. Mark Udall ­(D-Colo.), who indicated he would oppose Barron’s nomination unless the memo was released publicly.

“Barron’s nomination understandably raises key questions about the administration’s legal justification for the targeted killing of Americans and about its year-old pledge of greater transparency,” said Udall, a member of the Senate Intelligence Committee. He said the White House should comply with a recent court order “to release its redacted legal justification for killing a U.S. citizen.” He added, “Unless the White House complies, I cannot support David Barron’s nomination.”

Barron, a law professor at Harvard University, also faces opposition from Republicans, including Sen. Rand Paul (R-Ky.), who last year carried out a high-profile filibuster on the Senate floor protesting the secrecy surrounding executive branch decisions on killing Americans overseas.

Barron’s nomination to the appellate court in New England was approved by the Senate Judiciary Committee in January on a 10-to-8 vote but still requires approval from the full Senate. A committee aide said members have “seen reports of concern” about Barron’s nomination but that any decision to withdraw it would be made by the White House.

The White House has described the broad outlines of its drone policy but has withheld many details, citing executive privilege.

Last year, Attorney General Eric H. Holder Jr. sent a letter to the Senate Judiciary Committee saying the United States would seek to kill an American citizen overseas only as a last resort in cases when the targeted person posed “an imminent threat of violent attack against the United States” and capture was “not feasible.”

The controversy over Barron comes as the Justice Department is evaluating whether to approve a drone strike against a U.S. citizen in Pakistan accused of being part of al-Qaeda. U.S. officials have not identified the alleged al-Qaeda operative, and the deliberations over whether to add him to the administration’s “kill list” have come under criticism from Republicans.

Rep. Mike Rogers (R-Mich.), chairman of the House Intelligence Committee, complained during a hearing this year that “individuals who would have been previously removed from the battlefield by U.S. counterterrorism operations for attacking or plotting to attack against U.S. interests remain free because of self-imposed red tape.”

The 2011 strike on Awlaki, a hard-line Islamist preacher born in New Mexico, also killed a second American, Samir Khan, who was the editor of al-Qaeda’s online magazine, Inspire.