The conspiracy theorists who have claimed for more than a year that President Obama is not a United States citizen have found receptive ears among some mainstream media figures in recent weeks.

Despite ample evidence to the contrary, the country’s most popular talk radio host, Rush Limbaugh, told his listeners Tuesday that Mr. Obama “has yet to have to prove that he’s a citizen.” Lou Dobbs of CNN said that Mr. Obama should do more to dispel the claims. Larry King, also of CNN, asked guests about it, and other media types, including the MSNBC hosts Chris Matthews and Rachel Maddow, merrily mocked the controversy. NBC News even did a segment on the subject.

Cable news is often stretched for news in the summertime, but the birth certificate case has been fueled by the combustible combination of luck, compelling video, an outlandish topic, and savvy activists.

Advocates of the issue, who are sometimes called birthers, have succeeded in calling attention to themselves through frequent calls to talk radio shows, e-mail messages to news organizations and a videotaped question at a Congressional town hall. The Associated Press said that Mr. Obama’s citizenship status had been one of the most popular topics for the wire service’s “Ask AP” column.

Mr. Dobbs, who also hosts a daily radio show, said on CNN that he is “getting calls now. Often.”

The conspiracy theory that Mr. Obama was born in Kenya, his father’s home, first took root among some elements of the right wing last year. In response, the Obama campaign scanned the candidate’s “certification of live birth” from Hawaii’s department of health and published it on the Internet. It confirms that Mr. Obama was born in Honolulu on Aug. 4, 1961. Numerous third parties have examined the birth certificate and concluded, as the Annenberg Center’s nonpartisan FactCheck.org Web site did last year, that it “meets all of the requirements from the State Department for proving U.S. citizenship.”

In an e-mail message to Mr. Dobbs’ producers on Thursday, Jon Klein, the president of CNN/U.S., wrote that “it seems this story is dead — because anyone who still is not convinced doesn’t really have a legitimate beef.”

In an interview Friday, Mr. Klein said the e-mail message should not be interpreted as an order to stop debating the subject. He defended Mr. Dobbs’ broadcast, saying that “what we do here all the time is dig into the truth about all kinds of controversial issues.”

“This smear was thoroughly debunked during the election,” said Eric Burns, the president of Media Matters for America, a liberal media monitoring organization.

Remarkably, there is even a reference to Mr. Obama’s birth in the “Births, Marriages, Deaths” column of The Honolulu Advertiser newspaper on Aug. 13, 1961. Still, the claims about Mr. Obama’s citizenship persist among a small but vocal group, essentially portraying Mr. Obama as a foreigner who has managed to conceal his origins for nearly five decades.

“It’s racist,” said Phil Griffin, the president of MSNBC. “It’s racist. Just call it for what it is.”

Mr. Griffin said the cable news coverage was set off by two back-to-back events: the introduction of a bill that would require presidential candidates to provide a copy of their original birth certificate (it has nine co-sponsors in the House and one in the Senate) and the video of a town hall held by a Republican Congressman that turned into a hearing on Mr. Obama’s citizenship.

At the town hall in late June, the Congressman, Mike Castle of Delaware, was angrily questioned by a woman who said of Obama, “he is not an American citizen, he is a citizen of Kenya.” Members of the crowd applauded approvingly, and Mr. Castle was heckled when he said Mr. Obama was indeed a U.S. citizen. Video of the confrontation was uploaded to YouTube on July 10. It took more than a week for the incident to receive widespread attention.

Mr. Griffin said the claims are legitimate to cover “in that there’s a segment of our population that believes this and keeps bringing it up.”

A number of MSNBC shows have covered the birthers’ claims in recent days, mostly in mocking tones. On “Hardball” Thursday evening, the radio talk show host G. Gordon Liddy asserted that Mr. Obama may be an “illegal alien,” leading the host Chris Matthews to hand Mr. Liddy a photocopy of the birth certificate.

The subject has percolated on the Internet and on right-wing radio talk shows for months. The founder of WorldNetDaily, a conservative Web site, has taken to buying billboards that ask “Where’s the birth certificate?”

But from more mainstream outlets, “the coverage has been minuscule” until this month, said Philip J. Berg, a lawyer who has challenged Mr. Obama’s citizenship in a number of lawsuits, unsuccessfully to date. Since the town hall meeting, “there’s been a vast uptick.”

He added: “If I could file a lawsuit against the national media, including The New York Times, I would do that, because I think you’ve all done an injustice by not covering Obama.”

Since mid-July Mr. Dobbs has discussed it repeatedly on his radio show and on TV, leading Media Matters to accuse him of “legitimizing baseless conspiracy theories.” Last week CNN said it investigated the claims, and a guest host on Mr. Dobbs’ own show concluded that it “found no basis for the questions about the president’s birthplace.”

Mr. Dobbs has said he believes Mr. Obama is a citizen, but he brought up the controversy repeatedly this week, wondering in particular why Mr. Obama has not provided a copy of his birth certificate so that “all of this nonsense goes away.” He apparently answered his own question Thursday evening, telling viewers that “Hawaii says it can’t release a paper copy of the president’s original birth certificate because they say the state government discarded the original document when the health department records went electronic some eight years ago.”

The Fox News Channel, with its deep bench of conservative hosts, has not addressed the controversy this week. But the message boards on its conservative forum The Fox Nation have thousands of comments about the birth certificate issue, including some that call Mr. Obama “the Kenyan candidate.”

A lot of journalists “live with this issue; we get e-mails, we get asked about it,” Brian Williams, the anchor of the “NBC Nightly News,” said on Wednesday’s broadcast.

He showed an excerpt from the YouTube video in a segment about the birthers that he said were “spreading lies” about Mr. Obama.

“So many conspiracy theories, so little time,” he concluded.