A key member of the Supreme Court aimed a series of skeptical questions at opponents of race-conscious university admissions as the court heard oral arguments yesterday in the most important civil rights cases of the past quarter-century.

Justice Sandra Day O'Connor, widely viewed as holding the likely swing vote on an otherwise evenly divided court, seemed to take issue with the idea that University of Michigan admissions criteria intended to boost minority enrollment were necessarily to blame for the rejection of Barbara Grutter and other white students.

"How are we certain," she asked their attorney, "that there is an injury to your client that she wouldn't have experienced for other reasons?"

O'Connor said that their insistence on absolute color-blindness in admissions might be too inflexible. "You're speaking in absolutes, and it isn't quite that," she said. "I think we have given recognition to the use of race in a variety of settings."

Her remarks were among the most intriguing in a debate whose drama was commensurate with cases that could determine the consideration of race not only in college admissions but possibly in other realms as well.

As affirmative action faced its most important legal test since 1978, several thousand supporters of the concept rallied outside the Supreme Court building while the justices heard arguments in the two cases. Shortly after the arguments were completed, the court took the unusual step of making an audiotape of the proceedings publicly available. The last time the justices did that was in 2000, for the cases related to that year's presidential election.

At a time when the country's attention is occupied by the war in Iraq, several justices even implied that the country's national security might be affected by the outcome of the cases.

Two different admissions policies at the University of Michigan are at issue in the two cases that were argued yesterday, Grutter v. Bollinger, No. 02-241, and Gratz v. Bollinger, No. 02-516.

Grutter is a challenge to the university's law school admissions program, which gives African American, Latino and Native American applicants a loosely defined special consideration to ensure that there is a "critical mass" of such students in each new class.

Gratz is a challenge to the university's undergraduate admissions policy, which tries to ensure a "critical mass" of African American, Latino and Native American enrollments by giving such applicants an automatic 20-point bonus on the school's 150-point "selection index."

The university argues that its programs, which are similar to admissions policies at many other selective institutions, are designed to create educationally beneficial "diversity" in its student population. It says that this rationale was endorsed by the Supreme Court in the 1978 Bakke case.

In that case, the court ruled that universities could not set fixed numerical quotas for minority admissions as a remedy for past discrimination, but a separate opinion by Justice Lewis F. Powell Jr. left open the possibility that the schools could use race to achieve diversity in the hope of providing a better learning environment for all races, and universities have treated it as a binding precedent ever since.

Opponents of affirmative action contend that that position is far too vague and open-ended a notion to justify the violation of whites' clear constitutional right to equal treatment under the law. Admissions policies such as Michigan's are the practical equivalent of quotas, they argue.

Recent Supreme Court precedents have endorsed the notion that there is no constitutional distinction between racial discrimination intended to harm blacks and other minorities and programs intended to help them. Any racial classification, the court has held, must be carefully crafted to achieve a "compelling" government purpose, with the least possible harm to others' rights.

The Bush administration has attacked Michigan's programs on this "narrow tailoring" basis, arguing in a brief to the court that state universities must attempt race-neutral methods of ensuring diversity.

But until now, the court has not squarely revisited the issue of college and professional school admissions since Bakke.

The cases will affect affirmative action not only at state universities such as Michigan's but also private universities, since federal law forbids racial discrimination by institutions that receive federal funding, as most universities do.

National security considerations unexpectedly entered the debate yesterday when Justices Ruth Bader Ginsburg and John Paul Stevens, both clearly sympathetic to the Michigan program, pressed President Bush's solicitor general, Theodore B. Olson, to present his view on similar programs at the nation's military academies.

In a friend-of-the-court brief, retired Gen. H. Norman Schwarzkopf and other former top-ranking members of the armed forces argued that the fighting capability of the multiethnic U.S. military depends on minority officers produced through affirmative action at the service academies. Unstated, but obvious, was the broader context: the current battle for a Muslim country's liberation being waged by multiethnic U.S. Army and Marine platoons.

Speaking for an administration that both opposes the Michigan program and operates the service academies, Olson said that all institutions should seek race-neutral means to ensure diversity, playing down the officers' brief as "the opinion of certain individuals."

Ginsburg did not seem satisfied, noting that, by Olson's logic, the service academies' policies would be "illegal."

The opponents of affirmative action in the court scored points of their own.

Justice Antonin Scalia led Maureen Mahoney, a lawyer for the university, through an extended interrogation intended to show that the law school's "critical mass" policy -- which has produced minority enrollments ranging from 12 percent to 17 percent of the school's entering classes -- was just a euphemism for a quota. As Scalia put it, "once you use the terms 'critical mass,' you're in Quota Land."

"Your honor, what a quota is, under this court's cases, is a fixed number," Mahoney replied. "And there is no fixed number here."

That discussion gave way to what seemed to be O'Connor's most significant expression of concern about the Michigan policies. She noted that all previous affirmative action programs that the court has upheld were for a "fixed time period . . . you could see an end to it." In contrast, she said, Michigan seems to be advocating something open-ended.

"I don't think the court should conclude this is permanent," Mahoney answered, noting that educational institutions could conclude that there is no longer a need to use race-conscious admissions when minorities catch up with whites in grades and test scores, or when "we reach a point in society where the experience of being a minority no longer makes such a fundamental difference in their lives."

Perhaps sensing an opportunity to reinforce O'Connor's doubts, Scalia raised the same point later with John Payton, another lawyer for Michigan.

"When does all of this come to an end?" Scalia asked simply.

Payton acknowledged that "it was a surprise" that the gap between white and minority achievement has persisted so long after Bakke, creating what universities still consider a need for affirmative action 25 years later, but he added: "We're confident this is going to last for a finite period of time."

Payton, who is African American, spoke with evident emotion toward the end of his argument as he told the justices that diversity is crucial to ensure that students who would otherwise grow up in segregated surroundings can overcome stereotypes and "be able to do things together."

But Payton's effort was somewhat undercut by the court's only African American member, Clarence Thomas. Thomas asked Payton whether his view that diversity is crucial to a good education doesn't weaken the case for historically black colleges.

"Would the same arguments apply to them?" Thomas asked.

Payton said such institutions "do have diverse student bodies."

Decisions in the cases are expected by July.

Bryan Mason, left, Jeanine Hays and Jocelyn Stotts sing at a Lincoln Memorial rally backing affirmative action in schools. Story, Page B1.Solicitor General Theodore B. Olson is shown arguing for race-neutral admissions in the University of Michigan cases as Justices Ruth Bader Ginsburg, John Paul Stevens, Clarence Thomas and Stephen G. Breyer listen.Demonstrator Margaret Williams holds a sign addressed to Supreme Court Justice Clarence Thomas. The high court heard arguments yesterday in the affirmative action cases involving admissions at the University of Michigan.After holding a demonstration at the Supreme Court, affirmative action supporters walk toward the Lincoln Memorial for a second rally.