Bill Clinton was forced to appear on a black radio show for the second time in 24 hours yesterday to limit the fallout from remarks by one of his wife’s prominent African-American supporters.

As the Clinton and Obama race becomes increasingly ugly - with Barack Obama’s past drug use being thrown into the mix - the pair prepare to face each other tonight for the first time since the bitter dispute about race engulfed their presidential contest.

The rivals meet for a debate in Nevada before next week’s contest in South Carolina, their first showdown in the Deep South where half the state’s Democratic voters are black.

Mr Clinton took to the airwaves to try to explain inflammatory remarks made by Bob Johnson, the founder of Black Entertainment Television, at an event where he was introducing Mrs Clinton. Mr Johnson had said: “Barack Obama was doing something in the neighbourhood. I won’t say what he was doing, but he said it in his book.”

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

Multimedia

• Full coverage: US elections

BACKGROUND

• The ascent of Barack Obama, Mr Charisma

• Decay, jobs and guns fall under the spotlight

• Free trade: Clinton/Obama's mistakes

Related Links

• Romney digs up past to aim at the future

• Clinton gaffe may cost votes in South Carolina

• Dirt begins to fly at Obama

Multimedia

• Read our dedicated US elections blog

Mr Obama has written about his teenage drug abuse — marijuana and cocaine — in his memoir Dreams from My Father. Last month one of Mrs Clinton’s national chairmen was forced to resign after suggesting that Mr Obama’s drug use would be used against him by Republicans in a general election.

Mr Johnson later issued a statement saying he had only been referring to Mr Obama’s work as a community organiser in Chicago. Mr Clinton told the radio audience on a show popular with black listeners that “we should take him at his word” — a comment met with incredulity by his audience.

A spokesman for Mr Obama said Mr Johnson’s “tortured explanation” did not add up, adding: “It is troubling that neither the campaign nor Senator Clinton is willing to condemn it.”

The Obama campaign maintains that Mrs Clinton, through surrogates, is trying to use his past drug use as a way to damage Mr Obama.

The dispute began after Mrs Clinton said Martin Luther King’s dream of racial equality was realised only when President Johnson managed to get the Civil Rights Act of 1964 through Congress. She was making a point that it took presidential leadership to achieve Dr King’s ambition.

Mr Obama accused Mrs Clinton, who attended an event yesterday commemorating what would have been Dr King’s 79th birthday, of diminishing the civil rights leader’s legacy. They also accused Mr Clinton of being racially insensitive, after he said Mr Obama’s claim to have been consistently against the Iraq war was a “fairytale”.

Mrs Clinton hit back on Sunday, accusing Mr Obama’s campaign of “deliberately distorting” her remarks. Yesterday, her campaign pointed to a memo prepared by an Obama aide listing comments by Mrs Clinton and her surrogates that could be construed as racially insensitive.

Mr Obama said Mrs Clinton’s allegations were “ludicrous”, adding: “I think they have decided to run a relentlessly negative campaign, and I don’t think anybody who’s watching would deny that.”

The fact that the subject of race, the issue burnt most deeply into the American psyche, has erupted in the campaign is dismaying many Democrats, who fear it could become a corrosive force within the party and damage either candidate in the run-up to the general election.

To underscore such sensitivities, a new poll showed Mrs Clinton leading among white voters 41 per cent to 27, with Mr Obama enjoying a 66 per cent to 16 per cent among African-Americans. Another survey showed Mr Obama narrowing Mrs Clinton’s advantage nationally.

Times Recommends

• 'New York has lost its edge'
• Copenhagen will fail – and quite right too
• More than 17,000 new underwater species found