Democracy for Hong Kong, he said, is his “vocation.” But the struggle is taking a toll.

Mainland universities and, recently, a major newspaper in the southern province of Guangdong have withdrawn speaking invitations, the newspaper citing an “act of God.” This, Mr. Chan said, refers to political pressure that cannot be withstood. He has resigned as director of his university’s Chinese studies center and will soon give up another post as director of its Center for Civil Society Studies, he said.

“The university didn’t give me any pressure,” he said. “They want me to stay. But I knew it wasn’t for the benefit of the centers.” If he remained director, he feared, fellow academics and students would be frozen out of research activities in China.

The man who long prided himself on his moderation has become a persona non grata in most of his own country.

Why do it? Hong Kong, a sophisticated city, is becoming ungovernable, he said. Frustrations run high; people are unable to vote for representatives who can actually make policy and reflect their demands — the legislature is partly chosen by direct election, but partly through professions-based constituencies, and lacks real power. Issues like land use, housing, education and the environment are pressing, but power is concentrated in the person of the chief executive, who is chosen by the election committee with the tacit approval of Beijing and therefore is not seen as representative, he said.

Beijing is interested in good governance in Hong Kong and in preserving stability, said Mr. Chan, but he doesn’t believe it will allow its leaders to be elected by the public at large, whatever its promises.

“I am very pessimistic that they will give Hong Kong universal suffrage in 2017 unless we do something, such as Occupy Central,” he said.

So the true puzzle is perhaps this: Firmly opposed to democracy at home, why did Beijing excite these expectations in Hong Kong? Article 45 of the Basic Law, Hong Kong’s constitution adopted in 1997, says the “ultimate aim” is to choose a chief executive “by universal suffrage.” In 2007, the National People’s Congress announced the chief executive “may” be chosen by universal suffrage in 2017, and Hong Kong legislature “may” be fully directly elected in 2020.