WASHINGTON (Reuters) - Conservatives on the Supreme Court expressed strong doubts about the validity of a key part of the 1965 Voting Rights Act, signaling that there could be a majority to strike down the heart of the landmark law.

Justice Anthony Kennedy, the court's swing vote on racial issues, said that "times change" when weighing the provision that requires strict oversight of election laws in nine mainly Southern states.

The court's liberal justices mounted a spirited defense of the law and questioned how the court could strike it down in a case from Alabama, where they said so many problems of voting bias persist.

(Reporting by Lawrence Hurley and Joan Biskupic; Editing by Will Dunham)