U.S. Supreme Court conservatives on Wednesday sharply criticized a central part of the 1965 Voting Rights Act that is aimed at more than a dozen states with a history of racial discrimination.

It is the second major race case heard by the justices after Barack Obama became the nation's first black president.

The justices seemed split along conservative and liberal lines in considering a provision applying to all or parts of 16 states, mostly in the South. It requires them to get federal government approval before changing their voting procedures.

Congress adopted the Voting Rights Act, an historic piece of U.S. civil rights legislation, to make it easier for millions of blacks and other minorities to exercise their right to vote.

Congress extended it in 2006 for 25 years, with then-President George W. Bush signing it into law.

Last week the justices considered whether race still can be used as a factor for job promotions and hirings, an issue that could affect millions of employers nationwide.

Opponents of the voting rights law argue that the protections for minority voters are no longer needed after more than 40 years of progress, and they cite Obama's election as evidence of how America has changed since 1965.