US election 2012: Barack Obama 'birther' conspiracy theory revived by Republicans Mr Bennett's intervention, which will not be welcomed by Mr Romney's campaign, gave voice to a residual hard core of so-called "birthers" who refuse to believe Mr Obama is entitled to lead the US. A persistent campaign of misinformation on the subject, led at one stage by the celebrity property tycoon Donald Trump, last year forced Mr Obama to take the humiliating step of releasing a longer version of his birth certificate in a press conference days before the assassination of Osama bin Laden. A shorter version of the certificate was released during Mr Obama's first campaign for the White House, and his birth was reported at the time by two local newspapers in Hawaii. Yet Republicans in Iowa, a Midwestern state that Mr Romney is aggressively targeting, have also called into question Mr Obama's citizenship in what one party official described as "a shot" at the president. Their official platform for the 2012 election states that presidential hopefuls "must show proof of being a 'natural born citizen'," which is the constitutional requirement for a US presidential candidate. While the US Constitution does not specify precisely what being a "natural born citizen" entails, it has long been taken to mean someone who was born on American soil, even to foreign parents. However Don Racheter, the chairman of the Iowa Republicans' platform committee, told a radio interview that this no longer satisfied a large number of the president's opponents in the state. "There are many Republicans who feel that Barack Obama is not a 'natural born citizen' because his father was not an American when he was born and, therefore, feel that according to the Constitution he's not qualified to be president, should not have been allowed to be elected," Mr Racheter said. Mr Obama's father, Barack Obama sr., was born in Kenya and studied from 1959 to 1962 at the University of Hawaii, where he met the president's mother, Stanley Ann Dunham. The latest resurrection of "birtherism" came a week after a short biography published by Mr Obama's book publishers in the 1990s, stating that he was "born in Kenya," emerged on a news website. Mr Obama's former literary agent said that the president had not been responsible for the content of the biography and that it had simply not been fact-checked.