Bolstered by the promised public release of a secret legal memo, Senate Democrats are ready to approve a top federal judgeship for a former Obama administration official who helped formulate the justification for the drone killings of suspected American terrorists overseas.

The Senate was expected Thursday to approve David Barron to join the 1st U.S. Circuit Court of Appeals, which is based in Boston.

Barron cleared a pivotal hurdle Wednesday when senators voted 52-43 to end opponents' efforts to derail the nomination. In that roll call, Barron was opposed by every voting Republican and two Democrats, Sens. Mary Landrieu of Louisiana and Joe Manchin of West Virginia.

Barron's selection by President Barack Obama became a battleground over administration claims that it has constitutional authority to use unmanned, armed drones to kill Americans overseas who are believed to be terrorists. Lawmakers from both parties also criticized the administration for refusing to reveal the public documents explaining its rationale.

"Sections of the playbook for combatting terrorism will often need to be secret," said Sen. Ron Wyden, D-Ore. "But the rulebook that the United States follows should always be available to the American public."

The Harvard Law School professor has also drawn Republican fire as being too liberal.

At issue were an unknown number of memos that Barron wrote in 2009 and 2010, while acting head of the Justice Department's Office of Legal Counsel, providing the legal rationale for the drone attacks. Attorney General Eric Holder has said such killings are legal abroad if the American targeted poses an imminent threat to the U.S. and cannot be captured.

In 2011, a U.S. drone in Yemen killed American-born Anwar al-Awlaki, who administration officials say became an al-Qaida leader. Officials acknowledge three other Americans killed by drones but say they weren't specifically targeted.

Barron's nomination gained steam after the administration revealed Tuesday that it would not battle a federal appeals court order to release a censored version of one of his memos. That will occur only after the administration and the court determine what parts of the memo will be excised, but the decision to unveil it helped ensure support from Wyden, Colorado Sen. Mark Udall and perhaps others.

Sen. Rand Paul, R-Ky., a possible GOP presidential contender for 2016, initially was demanding the release of all drone documents by Barron. But Wednesday, he said the nomination should be defeated because killing Americans without a fair trial is unconstitutional.

"Do we have the courage to denounce drone executions as nothing more than sophisticated vigilantism?" said Paul.