Kerim Yildiz, a leading human rights advocate for the Kurdish people, was for two decades a frequent visitor to the United States. A British citizen living in London, he regularly lectured at American universities, caucused with other human rights advocates and briefed government officials in Washington.

But something changed. Where before he was admitted to the country without a problem, he has now waited nearly a year for the Obama administration to approve a visa. Officials have not explained the delay to him.

“It’s a surprise,” Mr. Yildiz said this week in a telephone interview from London, where he is executive director of the Kurdish Human Rights Project. “I know no reason why my application is still pending.”

Civil libertarians and human rights advocates in the United States have taken up his cause, and some suspect the government may be stalling for political reasons.

The case echoes those involving Tariq Ramadan, a prominent scholar of the Islamic world from Switzerland, and Adam Habib, a Muslim academic from South Africa, who were barred from entry by the Bush administration under the Patriot Act. Their advocates contended that the men had been singled out because of their political views and criticism of American foreign policy.

The American Civil Liberties Union sued on behalf of the two academics, arguing that the exclusions were an illegal form of censorship. Secretary of State Hillary Rodham Clinton lifted the bans last year.

Since then, the civil liberties group has lobbied on behalf of two other human rights advocates whose visitor visas were denied: Hollman Morris, a Colombian journalist, and Malalai Joya, an Afghan politician and writer. In those cases, the Obama administration quickly reversed course and issued the visas.

A State Department spokeswoman, citing confidentiality laws, said administration officials were forbidden to comment on Mr. Yildiz’s visa application, which he filed last fall.

“The longer the delay stretches out, the more we have to ask what the reasons for the delay might be,” said Jameel Jaffer, deputy legal director at the American Civil Liberties Union, which posted a notice about Mr. Yildiz’s case on its blog under the title “Ideological Exclusion Again?”

“I hope it doesn’t have to do with Mr. Yildiz’s human rights advocacy,” Mr. Jaffer continued. On Sept. 15, the civil liberties group, joined by the American Association of University Professors and the PEN American Center, sent a letter to Mrs. Clinton and Janet Napolitano, the homeland security secretary, urging them to expedite Mr. Yildiz’s visa.

“Permitting him to visit the United States would serve the public interest, particularly because his area of specialty — Kurdish rights — is one of obvious relevance to current public debates, including those about the United States’ relations with Iraq and Turkey,” the letter said.

Mr. Yildiz said his tentative itinerary in the United States included briefings with donors and supporters, a meeting with State Department officials to discuss Kurdish issues, lectures at City University of New York and Harvard, and a ceremony in Philadelphia, where his human rights organization is to receive a prize from the Peter and Patricia Gruber Foundation, a philanthropic group promoting social justice.

“This delay has effects on our human rights work,” Mr. Yildiz said. “Who knows how many people will be affected by this?”