WASHINGTON — The Obama administration and civil rights groups are defending a key section of the landmark voting rights law before the Supreme Court, insisting that reformed state, county, and local governments can use an escape hatch from the law’s strictest provision.

The Voting Rights Act effectively attacked persistent discrimination at the polls by keeping close watch, when it comes to holding elections, on those places with a history of preventing minorities from voting. Any changes, from moving a polling place to redrawing electoral districts, can’t take effect without approval from the Justice Department or federal judges in Washington.