Jeffrey Rosen’s puffer on the justice includes this passage:

Consider affirmative action, an area in which many people believe Stevens became more liberal. In 1980, he dissented from the court’s decision upholding racial preferences in federal contracting, explosively comparing them with the Nazi laws excluding Jews from citizenship. More recently, however, he has voted enthusiastically to uphold affirmative action in universities and public-school enrollment plans, comparing them to welcome mats rather than no-trespassing signs. To Stevens, however, his views have been consistent. “There’s a tremendous difference in using affirmative action when you get a group to build a highway and affirmative action in the educational context,” he told me. “I think my rhetoric was probably a little strong,” he continued, but the federal law authorizing racial preferences for highway contracts was a “slapdash statute” that was based on pork-barrel politics, benefiting one group of contractors rather than citizens as a whole. In schools and universities, by contrast, “the whole student body profits from having diversity in the classes. So I really don’t think I’ve changed my views about this.”
Stevens has been prescient in cases involving race discrimination. As early as 1948, when he was a law clerk, he wrote a memo to Rutledge suggesting that segregation was unconstitutional, as Diane Amann, a visiting professor of law at the University of California at Berkeley, has noted. And in 1986, he was the first justice to suggest that affirmative action for African-Americans should be justified not as a group-based remedy for past discrimination but as a way of securing the benefits of racial diversity for all citizens in the future. That insight informed his powerful dissent from Roberts’s 5-to-4 decision last June striking down the use of race in school enrollment. The notion that judges should treat all racial classifications alike, he told me, “doesn’t make any sense.”

But in 1978, Stevens wrote an opinion in Bakke rejecting, as illegal, racial preferences “in the educational context.” To my knowledge, he has never explained what was wrong with his reasoning in that opinion, which seems pretty solid. (Incidentally, the diversity rationale for racial preferences appears in Justice Powell’s opinion in Bakke.)