President Barack Obama says it will become harder to help people who believe their voting rights have been violated if the Supreme Court strikes down a key part of a voting rights law.

The court is set to hear arguments Wednesday. It's a challenge to a section of the law requiring states and local governments with a history of racial discrimination to get Justice Department approval before making changes that affect elections.

The appeal from Shelby County, Ala., argues that places covered by the law have made such progress that Washington oversight is unnecessary.

Defenders of the law say it's still needed.

Obama said Friday that ending federal oversight would stop people from challenging potential obstacles to voting before they are put in place.