The criticism from Sen Rand Paul, a Republican and possible presidential contender in 2016, came as the Senate moved toward a crucial procedural vote on David Barron, a former Justice Department official under Obama and President Bill Clinton. Obama has nominated Barron to the 1st US Circuit Court of Appeals, based in Boston.
The administration yesterday decided to release a censored version of one Barron memo in which he arguing that targeting an American in counterterrorism operations is constitutional.
"I cannot support and will not support a lifetime appointment of anyone who believes it's OK to kill an American citizen not involved in combat without a trial," Paul said in the Senate.
Paul noted that Obama said during his 2008 presidential campaign that he opposed Bush administration claims that presidents have the power to detain American citizens on charges of being enemy combatants.
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Several Democratic senators have insisted on the public release of Barron's documents on the killings. The decision to disclose one memo won over at least two senators needed by Democrats to confirm Barron.
Sen Ron Wyden, a Democrat, called the administration's move "clearly a very constructive step" and he said he would back Barron. A day earlier, Sen Mark Udall, a Democrat, announced he would support the nominee, now a Harvard Law School professor.
Democrats control 55 Senate votes. Under an easing of procedural rules they pushed through the chamber in November, Senate Majority Leader Harry Reid will need just a simple majority to end delaying tactics against Barron, not the 60 votes once necessary.
In 2009 and 2010, Barron was acting head of the Justice Department's Office of Legal Counsel.