WASHINGTON — Just four months after the Supreme Court largely ducked the question of whether the University of Texas may take account of race in its admissions decisions, the justices heard arguments on Tuesday about whether Michigan’s voters violated the Constitution by forbidding race-conscious admissions plans at that state’s public universities.
The new question led to a lively argument that illuminated the justices’ very different understandings of the meaning of equal protection, the workings of the political process and the value of affirmative action programs. The court appeared closely divided, with the five more conservative justices seeming generally inclined to uphold the ballot measure.
The measure, known as Proposal 2, was a response to Grutter v. Bollinger, a 2003 Supreme Court decision that upheld the use of race as one factor of many in law school admissions to ensure educational diversity. Proposal 2, approved in 2006 by 58 percent of Michigan’s voters, amended the State Constitution to prohibit discrimination or preferential treatment in public education, government contracting and public employment.
Groups favoring affirmative action sued to block the part of the law concerning higher education. The United States Court of Appeals for the Sixth Circuit, in Cincinnati, ruled last year that the initiative, which amended the State Constitution, violated the federal Constitution’s equal protection clause.
On Tuesday, John J. Bursch, Michigan’s solicitor general, told the justices that Proposal 2 was plainly constitutional. “It does not violate equal protection to require equal treatment,” he said.
But Mark D. Rosenbaum, a lawyer with the American Civil Liberties Union, which represents a group of applicants, students and faculty members, said Proposal 2 violated equal protection principles by restructuring the state’s political process and making it harder for disfavored minorities to press for change.
The court seemed divided along ideological lines.
“It’s not a racial classification,” Justice Antonin Scalia said of the ballot initiative. “It’s the prohibition of racial classifications.”
Justice Ruth Bader Ginsburg saw matters differently. “We can’t put hurdles in the way of a disadvantaged minority,” she said.
The lawyers, for their part, offered varying accounts of the impact of Proposal 2 and similar measures. Mr. Bursch said minority admissions in Michigan were affected only modestly in the year after Proposal 2 was adopted. Later statistics are unreliable, he said, because many students chose not to identify themselves as members of a single racial or ethnic group on a new form.
In California, which banned race-conscious admissions plans at the state’s public universities in 1996, “what they discovered was that underrepresented minority students have higher G.P.A.’s, that they take more technology, engineering and math classes, and they have a graduation rate that is 20 to 25 percent higher than it was before,” Mr. Bursch said. Texas, he said, saw similar trends in the years that affirmative action programs were barred there.
“What the California and Texas experiences have demonstrated is that there are good, positive reasons why the voters might want to try a race-neutral alternative,” Mr. Bursch said.
Chief Justice John G. Roberts Jr. went further, asking whether some minority students actually suffered a disadvantage by being admitted into academic settings where they could not thrive. He referred to research on this “mismatch” theory from Richard Sander and Stuart Taylor Jr., who had filed a brief supporting Michigan.
“Do we have to assume in your favor that these definitely are beneficial to particular minority groups?” Chief Justice Roberts asked, referring to race-conscious admissions plans.
Shanta Driver, a lawyer for the Coalition to Defend Affirmative Action, Integration and Immigrant Rights and Fight for Equality By Any Means Necessary, responded: “Certainly the minority voters of Michigan believe them to be, because 90 percent of black voters in Michigan voted against” the ballot measure there.
“If you look at the law schools, the medical schools, the professional schools now in the state of Michigan, there’s been a precipitous drop in underrepresented minority enrollment in those schools,” she continued. “We are going back to the resegregation of those schools because of the elimination of affirmative action.”
More generally, she said, the Michigan measure is unconstitutional because it fails “to protect minority rights against a white majority.”
The statement provoked a sharp response from Justice Scalia. “My goodness,” he said, “I thought we’ve held that the 14th Amendment protects all races. I mean, that was the argument in the early years, that it protected the blacks. But I thought we rejected that.”
In 1969 and 1982, the Supreme Court struck down measures disfavoring minorities on the grounds that they unfairly restructured the political process. Those precedents may be at risk in the new case, and much of the argument was devoted to their precise meaning and whether they could be reconciled with a decision endorsing the Michigan measure.
Chief Justice Roberts and Justices Anthony M. Kennedy and Samuel A. Alito Jr. all asked questions that tried to test what principle distinguishes an ordinary law from one that impermissibly restructures the political process. They did not seem satisfied with most of the responses they received.
Justice Sonia Sotomayor offered her own response: she said any measure that upset “normal academic decision making” was suspect.
Justice Alito did not seem persuaded by what he called “Justice Sotomayor’s answer to my question.” But he did propose another answer. “If you change the allocation of power in one of these less substantial ways, that’s one thing,” he said. “But when you require a constitutional amendment, that’s really a big deal.”
Should the Supreme Court strike down the Michigan measure, its ruling could also invalidate California’s ban, along with ones in Arizona, Florida, Oklahoma, Nebraska, New Hampshire and Washington.
Tuesday’s argument was heard by an eight-member court. Justice Elena Kagan recused herself, presumably because she had worked on the case as United States solicitor general. The federal government, which supported the University of Texas in the case decided in June, took no position in the new one, Schuette v. Coalition to Defend Affirmative Action, No. 12-682.