At the Supreme Court today, the conservative justices had skeptical questions for a lawyer defending the University of Texas' plan that takes race into consideration in the admissions process.

One of their main concerns goes to the heart of the case: at what point does the court stop deferring to a university's judgment that the consideration of race is still necessary?

"I understand my job under our precedents is to determine if your use of race is narrowly tailored to a compelling interest, "Chief Justice John Roberts said to Gregory Garre, a lawyer representing the University of Texas. "The compelling interest you identify is attaining a critical mass of minority students at the University of Texas, but you won't tell what the critical mass is. How am I supposed to do the job that our precedents say I should do?"

It was only recently, in 2003, that the Supreme Court narrowly upheld the limited use of race in public universities in a decision called Grutter v. Bollinger. The 5-4 opinion was authored by Justice Sandra Day O'Connor, who said that diversity was a compelling government interest.

But today, O'Connor was sitting in the audience, in the front row. She has retired from the bench and Justice Samuel Alito, who took her seat, is more skeptical of racial preferences.

In the Grutter decision, O'Connor said that the University of Michigan Law School could use race in a "flexible nonmechanic way" as one factor among many in an effort to assemble a student body that is diverse in ways broader than race. "We expect," O'Connor wrote, "that 25 years from now, the use or racial preferences will no longer be necessary."

Garre said that Texas had closely followed Grutter when it implemented its own program with a goal of assembling "a broadly diverse student body."

Chief Justice John Roberts broke in: "Should someone who is one-quarter Hispanic check the Hispanic box or some different box? "

As Garre tried to explain the various boxes that students check on their own determination, Roberts pressed on.

"What about one eighth?" Roberts was getting to the issue of race and building toward his central question and his frustration with a definition for "critical mass."

"What is the critical mass of African Americans and Hispanics at the university that you are working toward?" Roberts asked

Garre responded that there was no hard number.

Justice Alito said, "Does critical mass vary from group to group? Does it vary from state to state?"

"How are we supposed to tell whether this plan is narrowly tailored to that goal?" Roberts asked.

None of this was a good sign for the University of Texas plan, which is being challenged by Abigail Fisher, a white Texan who was denied admission to the school in 2008. She argues that she was subject to unequal treatment in violation of the Fourteenth Amendment.

Justice Alito asked about African American and Hispanic applicants who come from privileged backgrounds. "They deserve a leg-up against, let's say, an Asian or white applicant whose parents are absolutely average in terms of education and income? " he asked

Garre said, "Our point is, that we want minorities from different backgrounds. We go out of our way to recruit minorities from disadvantaged backgrounds."

That answer energized Justice Anthony Kennedy. Kennedy's vote is seen as crucial to this case. In the past he has said that there is no constitutional objection to the goal of considering race as "one modest factor among many others to achieve diversity," but he cautioned that each applicant must receive individual consideration so that race does not become a predominant factor in the admissions decision making. Kennedy has never voted in favor of a racial preference.

Kennedy said to Garre, "So what you're saying is that what counts is race above all."

Garre responded, "What we want is different experiences that are going to -"

But Kennedy cut him off. "You want underprivileged of a certain race and privileged of a certain race. So that's race."

The liberal justices seemed much more accepting of UT's plan. Justice Ruth Bader Ginsburg said, "This program is certainly no more aggressive than the one in Grutter; it's more - in fact, more modest."

Justice Stephen Breyer pointed out how recently the court upheld Grutter. "Grutter said it would be good law for at least 25 years, and I know that time flies, but I think only nine of those years have passed. "

Justice Sonia Sotomayor asked the most questions of Bert Rein, the lawyer representing Fisher. Near the end of the arguments she told him that it seemed he didn't want to overrule Grutter, "but you just want to gut it."

Justice Elena Kagan was recused from the case; she may have dealt with it in her previous job as Solicitor General.