WASHINGTON — I BELIEVE that killing an American citizen without a trial is an extraordinary concept and deserves serious debate. I can’t imagine appointing someone to the federal bench, one level below the Supreme Court, without fully understanding that person’s views concerning the extrajudicial killing of American citizens.

But President Obama is seeking to do just that. He has nominated David J. Barron, a Harvard law professor and a former acting assistant attorney general, to a seat on the United States Court of Appeals for the First Circuit.

While he was an official in the Justice Department’s Office of Legal Counsel, Mr. Barron wrote at least two legal memos justifying the execution without a trial of an American citizen abroad. Now Mr. Obama is refusing to share that legal argument with the American people.

On April 30, I wrote to the Senate majority leader, Harry Reid, urging him to delay this nomination, pending a court-ordered disclosure of the first memo I knew about. Since that letter, I have learned more. The American Civil Liberties Union sent a letter to all senators on May 6, noting that in the view of the Senate Intelligence Committee chairwoman, Dianne Feinstein, “there are at least eleven OLC opinions on the targeted killing or drone program.” It has not been established whether Mr. Barron wrote all those memos, but we do know that his controversial classified opinions provided the president with a legal argument and justification to target an American citizen for execution without a trial by jury or due process.