WASHINGTON—The Supreme Court on Tuesday upheld Michigan's decision to end affirmative action at its public universities in a 6-2 ruling, but the justices were divided in their reasoning, suggesting continued uncertainty over the broader issue of racial preferences.

The ruling leaves in place a 2006 Michigan ballot initiative where voters ended race-based admissions at state schools, and means racial preferences won't soon return to the University of Michigan—or any other public university in states that have chosen to end the practice.

"Democracy does not presume that some subjects are either too divisive or too profound for public debate," Justice Anthony Kennedy wrote in backing the law.

The court's ruling didn't alter the ability of universities in states without bans to consider race as one factor among others in admissions. Instead, the court chipped away at affirmative action by giving its blessing to one path for foes to challenge admissions policies: ballot initiatives. Opponents have also gone to courts and state legislatures to end affirmative-action practices in a decadeslong battle over university policies.

Eight states, including California, have ended affirmative action since 1996. Practices vary widely among institutions. The higher-education establishment generally favors the use of racial preferences to promote diversity in the student body. Many of the nation's most selective universities—including the Ivy League, the U.S. military academies and flagship public institutions such as the University of Texas at Austin and the University of North Carolina at Chapel Hill—employ affirmative action.