WASHINGTON (AP) – The most potent weapon in fighting discrimination at the U.S. ballot box comes before the Supreme Court on Wednesday in a case that weighs the nation’s enormous progress in civil rights against the need to continue to protect minority voters.

The justices are hearing arguments in a challenge to the part of the Voting Rights Act that forces places with a history of discrimination, mainly in the Deep South, to get approval before they make any change in the way elections are held.

The advance approval was adopted in the Voting Rights Act in 1965 to give federal officials a way to get ahead of persistent efforts to keep blacks from voting. The provision was a huge success, and Congress periodically has renewed it over the years, most recently in 2006, when a Republican-led Congress overwhelmingly approved and President George W. Bush signed a 25-year extension.

Coverage has been triggered by past discrimination not only against blacks, but also against American Indians, Asian-Americans, Alaskan Natives and Hispanics.

But the lawsuit from Shelby County, Alabama, says the "dire local conditions" that once justified strict federal oversight of elections no longer exist.

The Obama administration and civil rights groups acknowledge the progress, but they also argue that Congress was justified in maintaining the advance approval provision when the law was last renewed in 2006.

Last week, President Barack Obama weighed in on behalf of the law in a radio interview with SiriusXM host Joe Madison. "It would be hard for us to catch those things up front to make sure that elections are done in an equitable way" if the need for advance approval from the Justice Department or federal judges in Washington were stripped away, Obama said.

Advance approval has been successful because it requires the governments to demonstrate that their proposed election changes will not discriminate, the law’s advocates say. "It moved the burden from victims to perpetrators," said Sherrilyn Ifill, the head of the National Association for the Advancement of Colored People Legal Defense & Educational Fund.

But lawyer Bert Rein for Shelby County argues that Congress overstepped its authority when it renewed the law and its formula that relied on 40-year-old data, without taking account of dramatic increases in the voter registration and participation by minorities, or of problems in places not covered by the law.

The Supreme Court already has expressed deep skepticism about the ongoing need for the law. In 2009, the justices heard a similar challenge in which Chief Justice John Roberts wrote for the court that the law’s past success "is not adequate justification to retain the preclearance requirements." But the court sidestepped the question of the law’s constitutionality at the time.

The advance approval requirement currently applies to the states of Alabama, Alaska, Arizona, Georgia, Louisiana, Mississippi, South Carolina, Texas and Virginia. It also covers certain counties in California, Florida, New York, North Carolina and South Dakota, and some local jurisdictions in Michigan and New Hampshire.

Among the covered states, Alabama, Alaska, Arizona, Georgia, South Carolina, South Dakota and Texas are siding with Shelby County, while California, Mississippi, New York and North Carolina argue that the law should be upheld.