Justice Department lawyers concluded in an unpublished opinion earlier this year that the historic D.C. voting rights bill pending in Congress is unconstitutional, according to sources briefed on the issue. But Attorney General Eric H. Holder Jr., who supports the measure, ordered up a second opinion from other lawyers in his department and determined that the legislation would pass muster.

A finding that the voting rights bill runs afoul of the Constitution could complicate an upcoming House vote and make the measure more vulnerable to a legal challenge that probably would reach the Supreme Court if it is enacted. The bill, which would give the District a vote in the House for the first time, appeared to be on the verge of passing last month before stalling when pro-gun legislators tried to attach an amendment weakening city gun laws. Supporters say it could reach the House floor in May.