Mr. Obama led a march across the bridge on Saturday, after a speech in which he called the Voting Rights Act “the culmination of so much blood and sweat and tears, the product of so much sacrifice in the face of wanton violence.” The law, he said, “stands weakened, its future subject to partisan rancor.”

Congress, he said, should “pledge to make it their mission to restore that law this year.”

That could be difficult, given the highly partisan tenor of the debate over voting rights. In many states, particularly since 2010, Republicans have passed new laws requiring voters to show identification cards and restricting early-voting hours, arguing that these methods would help prevent voter fraud.