WASHINGTON — Eleven senators sent a letter to President Barack Obama on Monday demanding access to secret legal memos outlining the administration’s case for the targeted killing of U.S. citizens in counterterrorism operations overseas.

The letter from eight Democrats and three Republicans contained the most forceful warning to date that lawmakers were considering blocking Obama’s nominees to run the CIA and Pentagon unless the memos are turned over.

Three members of the intelligence panel signed the letter: Sens. Susan Collins, R-Maine, Ron Wyden, D-Ore., and Mark Udall, D-Colo.

The message comes three days before White House counterterrorism adviser John Brennan will face a confirmation hearing to become CIA director before the Senate intelligence committee. Former Sen. Chuck Hagel, R-Neb., is awaiting a vote in the Senate Armed Services Committee following his hearing last week to become secretary of defense.

Cooperation with the request “will help avoid an unnecessary confrontation that could affect the Senate’s consideration of nominees for national security positions,” the letter said.

In an interview, Wyden accused the administration of stonewalling. “The idea that the president has this extraordinary power that can be utilized in secret without any oversight or accountability, I think is wrong and detrimental to the public interest,” he said.

He stopped short of threatening to place a hold on Brennan’s nomination — a procedural step to prevent a vote.

Other lawmakers who signed the letter included Sen. Patrick Leahy, D-Vt., the chairman of the Senate Judiciary Committee, and Sen. Charles Grassley, Iowa, the ranking Republican on that panel.

Some key lawmakers did not sign, including Sen. Dianne Feinstein, D-Calif., the chairman of the intelligence committee, who has indicated that she will support Brennan.

The request is the latest attempt to force the White House to turn over secret legal opinions that lay out the rationales for killing Americans accused of taking part in terrorism plots.

The administration has cited executive privilege and attorney-client relationship between the White House and the Justice Department, which produced the memos.

The administration has briefed lawmakers on the memos and summarized their contents in writing, congressional officials said. But Wyden said the refusal to furnish actual documents has made a “mockery” of congressional oversight.

The only U.S. citizen targeted under the new authority was Anwar al-Awlaki, a U.S.-born cleric accused of helping an al-Qaida affiliate in Yemen carry out attacks against the United States. Three other Americans, including Awlaki’s teenage son, also have been killed in strikes but were not the primary targets.

In their letter, the senators said they don’t dispute that the president in extreme cases may need to use lethal force against Americans “just as President Lincoln had the authority to direct Union troops to fire upon Confederate forces.”

But the lawmakers cited the importance of having a “full understanding of how the executive branch interprets the limits and boundaries of this authority.”