On the weekend, protests organised by anti-big government Tea Party groups drew thousands to Washington bearing inflammatory banners, including one that portrayed Mr Obama as a witch-doctor, and others that said ''I want my country back ''.

In a network television interview, Mr Carter, who famously comes from Georgia and speaks with a Southern lilt, said he believed the anger evident among Mr Obama's opponents was because he was a black man.

''I think an overwhelming portion of the intensely demonstrated animosity towards President Barack Obama is based on the fact that he is a black man, that he's African-American,'' Mr Carter said.

''I live in the South, and I've seen the South come a long way. And I've seen the rest of the country that shared the South's attitude towards minority groups at that time, particularly African-Americans, and that racism inclination still exists.

''And I think it's bubbled up to the surface because of the belief among many white people, not just in the South but around the country, that African-Americans are not qualified to lead this great country. It's an abominable circumstance, and it grieves me and concerns me very deeply.''

Mr Carter was being interviewed for his 85th birthday for a segment to be screened at a later date, but NBC decided to air the remarks on racism immediately on Tuesday night as the debate in the US intensified. Mr Carter made similar remarks later at a town hall meeting.

Over the weekend tens of thousands of people attended rallies in Washington, which were notable for the hostile tone of their signs and posters.

One of the organisers of the Tea Party coalition, Mark Williams, who has variously described Mr Obama on his website as an ''Indonesian Muslim'' and a ''Nazi'', said just a small fringe element was responsible. But in Congress there are fears that racism has reared its ugly head again.