WASHINGTON — President Obama’s choice for a powerful appeals court appointment is in peril from both the left and the right, highlighting how the fraught politics of an election year are threatening the president’s agenda even among his allies on Capitol Hill.

The nomination of David Barron, who was a Justice Department lawyer at the start of the administration and is now a Harvard Law School professor, is mired in a maw of contentious issues. Republicans object to what they say are his radically liberal views on the Constitution. Democrats in conservative-leaning states, especially those who are up for re-election, are wary that a vote for him might backfire with voters at home. And members of both parties say they are disturbed by Mr. Barron’s authorship of legal memos that justified the United States’ killing of an American citizen overseas with a drone.

The American Civil Liberties Union wrote to all 100 senators on Monday urging them to put off a vote on Mr. Barron’s confirmation until the White House allowed them to read all of his writings on the drone program.