In a decision that likely makes California’s Proposition 209 bullet proof against legal attack, the U.S. Supreme Court on Tuesday upheld an identical Michigan law that bans affirmative action in public programs such as university admissions.

The Supreme Court, although splintered in its legal reasoning, determined that voters had a right to enact Michigan’s law in 2006 and choose to outlaw race and gender preferences. The majority decision, written by Justice Anthony Kennedy, based the outcome on a states’ rights argument, steering clear of whether Michigan’s law violated the constitutional rights of minorities in the state.

The case had major implications for California, where Proposition 209 since 1996 has forbidden consideration of race and gender in university admissions, contracting and other public programs throughout the state. Legal challenges to Proposition 209 have failed, and its critics considered the Supreme Court case out of Michigan the last, best chance to revive the prospect of a renewed challenge in the courts.

As a result of Tuesday’s decision, the only likely method for Proposition 209 opponents to get rid of the law would be a move to repeal it at the ballot box. However, that tactic has repeatedly stalled, including a recent proposal to put the issue before voters again that failed to rally enough support in the Legislature.

In Tuesday’s decision, the Supreme Court essentially upheld Michigan’s law by a 6-2 vote. But just three of the justices backed Kennedy’s approach, which was to conclude there is no constitutional reason to interfere with the right of Michigan voters to ban affirmative action.

“This case is not about the constitutionality, or the merits, of race-conscious admissions policies in higher education,” Kennedy wrote.

Justice Stephen Breyer agreed only with the result in the case, while Justices Antonin Scalia and Clarence Thomas would have gone further, saying Michigan’s approach to affirmative action is constitutional. Justices Sonia Sotomayor and Ruth Bader Ginsburg dissented. Justice Elena Kagan did not participate in the case.

California, joined by five other states and the District of Columbia, joined civil rights groups in asking the Supreme Court to invalidate Michigan’s Proposal 2, enacted in 2006. The University of California also opposed the law in the high court, noting that Proposition 209 has dramatically reduced the admissions rates for Latino, black and Native American students, particularly at highly selective schools such as UC-Berkeley and UCLA.

Other California interests sided against the Michigan law in the Supreme Court, including a number of high school districts in urban areas such as Berkeley, Oakland and San Francisco, where school officials say Proposition 209 dashes the college hopes of minority students. BART also jumped into the case, arguing that the state law has hampered hiring of minority contractors.

Legal experts had predicted that the conservative Supreme Court was likely to leave laws such as Michigan’s and California’s intact. Supporters of the law, such as Ward Connerly, a former UC regent and architect of the Michigan and California laws, argue that the approach is race-neutral, and does not target any particular group.

Civil rights groups decried the ruling, saying leaving the anti-affirmative action laws intact in states such as California and Michigan will damage efforts to diversify higher education.

“This case is ultimately about whether students of color in Michigan are allowed to compete on the same playing field as all other students,” said Mark Rosenbaum, an American Civil Liberties Union attorney who challenged both the Michigan and California laws. “Today, the Supreme Court said they are not.”

Connerly praised the Supreme Court decision, saying it forecloses further challenges to Proposition 209 in California.

“I believe (the ruling) brings finality … to Proposal 2 and Prop 209,” he said Tuesday.

Howard Mintz covers legal affairs. Contact him at 408-286-0236 or follow him at Twitter.com/hmintz