John Kamm, a veteran American human rights campaigner, said that during three decades working in China he had rarely seen such a hard line toward dissidents  and unbridled defiance against pressure from abroad. “China right now doesn’t feel like it owes anyone anything on human rights,” said Mr. Kamm, the founder of the Dui Hua Foundation, which seeks clemency for political prisoners through quiet diplomacy. “I’ve never seen a downward spiral like this.”

In the 31 years since the People’s Republic of China and the United States established diplomatic relations, Chinese officials have often resisted American intervention on human rights, calling the issue a domestic matter. But there has generally been some give and take, largely behind the scenes, especially in the years after the violent suppression of protests in Tiananmen Square, when China was eager to shed its pariah status abroad.

That leverage began dissipating in 2001 after China was admitted to the World Trade Organization, and Congress surrendered the right to review China’s human rights record before granting it favorable trade status.

There is little space in Chinese society for unyielding dissidents like Mr. Gao. But until recently, the authorities often allowed them to stay at home under close surveillance. If they crossed certain unwritten lines, they might be prosecuted, often for the crime of inciting subversion or leaking state secrets. Even if stymied in their defense, lawyers can expect a modicum of information about their clients. Family jailhouse visits are not uncommon.

But Mr. Gao’s case has defied these norms.

In September, a security agent who took Mr. Gao into custody told one of his brothers that he had simply disappeared during a walk. The brother, Gao Zhiyi, said he suspected the worst. “If he were alive, they would have allowed me to visit him,” he said in a telephone interview from his home in Shaanxi Province. “Either that or he’s in such bad shape, it would be too horrible for anyone to see him.”

Image In December, China executed Akmal Shaikh, above, a Briton, despite appeals by Britain. Credit... Luis Belmonte Diaz/Reprieve, via Reuters

Rights advocates say Mr. Gao’s predicament can be partly traced to his persistent and caustic criticism of the ruling Communist Party. A self-educated lawyer, Mr. Gao, 46, was named one of China’s top 10 lawyers by the Ministry of Justice in 2001 for his work defending victims of medical malpractice and farmers whose land had been seized for redevelopment.