KYOTO, Japan — For Japan, the crowning of Hiromu Nonaka as its top leader would have been as significant as America’s election of its first black president.

Despite being the descendant of a feudal class of outcasts, who are known as buraku and still face social discrimination, Mr. Nonaka had dexterously occupied top posts in Japan’s governing party and served as the government’s No. 2 official. The next logical step, by 2001, was to become prime minister. Allies urged him on.

But not everyone inside the party was ready for a leader of buraku origin. At least one, Taro Aso, Japan’s current prime minister, made his views clear to his closest associates in a closed-door meeting in 2001.

“Are we really going to let those people take over the leadership of Japan?” Mr. Aso said, according to Hisaoki Kamei, a politician who attended the meeting.

Mr. Kamei said he remembered thinking at the time that “it was inappropriate to say such a thing.” But he and the others in the room let the matter drop, he said, adding, “We never imagined that the remark would leak outside.”

But it did — spreading rapidly among the nation’s political and buraku circles. And more recently, as Mr. Aso became prime minister just weeks before President-elect Barack Obama’s victory, the comment has become a touchstone for many buraku.

How far have they come since Japan began carrying out affirmative action policies for the buraku four decades ago, mirroring the American civil rights movement? If the United States, the yardstick for Japan, could elect a black president, could there be a buraku prime minister here?

The questions were not raised in the society at large, however. The topic of the buraku remains Japan’s biggest taboo, rarely entering private conversations and virtually ignored by the media.

The buraku — ethnically indistinguishable from other Japanese — are descendants of Japanese who, according to Buddhist beliefs, performed tasks considered unclean. Slaughterers, undertakers, executioners and town guards, they were called eta, which means defiled mass, or hinin, nonhuman. Forced to wear telltale clothing, they were segregated into their own neighborhoods.

The oldest buraku neighborhoods are believed to be here in Kyoto, the ancient capital, and date back a millennium. That those neighborhoods survive to this day and that the outcasts’ descendants are still subject to prejudice speak to Japan’s obsession with its past and its inability to overcome it.

Yet nearly identical groups of outcasts remain in a few other places in Asia, like Tibet and Nepal, with the same Buddhist background; they have disappeared only in South Korea, not because prejudice vanished, but because decades of colonialism, war and division made it impossible to identify the outcasts there.

In Japan, every person has a family register that is kept in local town halls and that, with some extrapolation, reveals ancestral birthplaces. Families and companies widely checked birthplaces to ferret out buraku among potential hires or marriage partners until a generation ago. The practice has greatly declined, though, especially among the young.

The buraku were officially liberated in 1871, just a few years after the 13th Amendment abolished slavery in the United States. But as the buraku’s living standards and education levels remained far below national averages, the Japanese government, under pressure from buraku liberation groups, passed a special law to improve conditions for the buraku in 1969. By the time the law expired in 2002, Japan had reportedly spent about $175 billion on affirmative action programs for the buraku.

Confronting Prejudice

Fumie Tanaka, now 39, was born just as the special measures law for the buraku went into effect. She grew up in the Nishinari ward of Osaka, in one of the 48 neighborhoods that were officially designated as buraku areas.

At her neighborhood school, the children began learning about discrimination against the buraku early on. The thinking in Osaka was to confront discrimination head on: the problem lay not with the buraku but with those who harbored prejudice.

Instead of hiding their roots, children were encouraged to “come out,” sometimes by wearing buraku sashes, a practice that Osaka discontinued early this decade but that survives in the countryside.