In a campaign conference call former New Hampshire governor John Sununu, a supporter of Obama's challenger Mitt Romney, made a jibe that risked recalling previous attacks on the incumbent's racial and religious background.

Obama's camp jumped on the remark, alleging it represented a desperate move by Romney to stir up controversy and distract attention from criticism of the Republican candidate's tax records and business affairs.

"This meltdown and over-the-top rhetoric won't make things better. It only calls attention to how desperate they are to change the conversation," Obama campaign spokeswoman Lis Smith said in a statement.

Tensions were already sky-high between the two campaign teams, with negative ads and attacks flying in both directions.

Obama's crew has accused Romney of lying when he said he no longer led his investment firm Bain Capital when it was linked to large-scale industrial job losses between 1999 and 2002.

Democrats also question why the multimillionaire has offshore accounts and refuses to reveal his tax returns, saying they would show he pays a relatively low effective rate on his income thanks to canny accountancy.

"The question is what else they'll pull to avoid answering serious questions," Smith said.

Romney and his team have used the same argument against Obama, saying he prefers to engage in personal attacks rather than defend his administration's poor record on job creation.

Sununu's jab at Obama's American identity echoed those of conspiracy theorists and certain right-wing Republicans who question whether Obama was born on US soil, a constitutional requirement for any US president.

The White House in 2011 tried to put an end to the contentions, going as far as publishing Obama's complete birth certificate, showing he was born August 4, 1961 in the Pacific US state of Hawaii.