It's not because the president is black, of course.

It's because those upstanding Americans who cheered as Barack Obama's predecessor rode roughshod over the constitution in his war on terror have found a new enthusiasm for a strict adherence to the US's supreme law. Specifically they're interested in a clause requiring the president to be born a natural born citizen (although that doesn't mean to say they're not still worried about Obama also being a secret Muslim).

A long brewing conspiracy theory has it that Barack Obama entered this world as a subject of the British crown in East Africa because his father was Kenyan. A Hawaii birth certificate and birth notices in the Honolulu press went some way to dampen down the feverish speculation when it first emerged during Obama's election campaign, driven by a handful of rightwingers who helped scupper John Kerry's bid for president.

But now the issue has returned with a vengeance driven in part by a high profile CNN presenter, rightwing talk radio and a video of a woman haranguing her Republican member of congress prompting her supporters to recite the Pledge of Allegiance. Now members of congress are sponsoring a bill to require all future presidential candidates to show their birth certificates.

At the heart of the supposed conspiracy is Obama's failure to produce a paper version of his birth certificate because Hawaii digitalised its original records some years ago and now provides a print out of the electronic record. That print out shows he was born in Honolulu in 1961, a fact that was verified again today by the state's health director, Dr Chiyome Fukino. He said: "I ... have seen the original vital records maintained on file by the Hawaii State Department of Health verifying Barack Hussein Obama was born in Hawaii and is a natural-born American citizen."

But that is not good enough for what has become known as the birther movement which would have the world believe that Obama was born in Kenya and smuggled in to the country by his American mother, or some variation on that theme.

In recent days the issue has been fired up again by television and radio. The CNN news presenter, Lou Dobbs, who is openly hostile to the new administration, drew attention when he told viewers that the question of Obama's place of birth "hasn't been dealt with". The influential right wing talk show host, Rush Limbaugh, told listeners last week that the president ‚ "has yet to have to prove that he's a citizen".

But the real impact has been a video that has garnered hundreds of thousands of hits on the web. It shows a Republican congressman, Mike Castle, addressing a town hall meeting on health care in Delaware last month when a woman suddenly stands up waving a bunch of papers. She says this is her birth certificate and demands to see the president's. "He is not an American citizen, he is a citizen of Kenya," she shouts to applause from others in the audience. Castle insists that Obama was indeed born an American. The crowd boos. As the congressman tries to change the subject, the woman suddenly demands everyone recites the Pledge of Allegiance. The entire hall stands, faces the US flag, place their right hand on their hearts and begins reciting the pledge.

The encounter was a warning to Republican officials how far the conspiracy theory has permeated parts of their party. The incident reflected an undercurrent of suspicion about Obama among those who see him as somehow un-American because of his politics or race, aside from the theory that he is a secret Muslim because his middle name is Hussein. Other Republicans have taken note: Ten members of Congress are sponsoring legislation to force future presidential candidates to find their birth certificates - widely seen as a tacit endorsement of the conspiracy theorists.

One of the sponsors, John Campbell, was asked in a television interview if he believes the president is an American and was mocked for replying: "As far as I know." Other members of Congress have been hit with wads of "evidence" and demands for meetings by supporters of the birther movement.

The origins of the birther movement are murky but among those who played a leading role in getting it noticed is Andy Martin, a former Republican congressional candidate and open antisemite who has denounced Jews for secretly controlling the country. He helped drive the rumours that Obama was a secret Muslim who renounced US citizenship at the age of 10 while living in Indonesia. Martin filed a lawsuit last year demanding the Hawaii authorities verify his birth certificate.

There is also Jerome Cors who co-authored the Swift Boat book that questioned John Kerry's war record when he was running for president. Cors claims Obama's birth certificate is a forgery. He also writes for World Net Daily, a supposed news site that offers for sale bumper stickers demanding to know "where's the birth certificate?" and a DVD called A Question Of Eligibility: Is Obama's Presidency Constitutionally Legitimate?

Orly Taitz, a Russian born resident of California who describes herself as an attorney and dentist, has come to the forefront as an agitator on her blog and at meetings.

The tone of the questioning has raised unease at major networks. Dobbs' own producers have expressed concern over his repeated dwelling on the question of Obama's origins.

Joe Klein, president of the US wing of CNN, wrote to them to say that "it seems this story is dead — because anyone who still is not convinced doesn't really have a legitimate beef". But Klein later said he wasn't ordering Dobbs to stop talking about it.

The president of MSNBC, Phil Griffin, took a tougher line. He told the New York Times that the questioning is driven by the fact that America has elected a black president: "It's racist. Just call it for what it is."