WASHINGTON — The Supreme Court’s conservative justices voiced deep skepticism Wednesday about a section of a landmark civil rights law that has helped millions of Americans exercise their right to vote. In a fast-paced, 70-minute argument, the court’s liberals and conservatives engaged in a sometimes tense back and forth over whether there is an ongoing need in 2013 for a key provision of the Voting Rights Act of 1965. The measure requires states with a history of discrimination, mainly in the Deep South, to get approval before making changes in the way elections are held.