Hillary Clinton and Barack Obama have called a truce over a bitter dispute about race as they prepare to face each other in a Las Vegas debate tonight - but too late to stop the controversy dividing the Democratic party.

Mr Obama, seeking to become America's first black president, was the first to try to defuse what had become an increasingly corrosive debate, calling reporters together to say he didn't want the campaign "to degenerate into so much tit-for-tat, back-and-forth, that we lose sight of why all of us are doing this".

Referring to Mrs Clinton and John Edwards, the other leading Democrat still in the race, he said they may have disagreements, but "we share the same goals. We're all Democrats, we all believe in civil rights, we all believe in equal rights." Mr Obama made his remarks after the issue of his past - and acknowledged - drug use was once again dragged into the contest by a Clinton supporter.

About an hour afterwards, Mrs Clinton issued a statement calling for "common ground" after the dispute had engulfed not only their presidential battle, but was also threatening to harm the Democratic party's overall drive to win the White House in November.

"We differ on a lot of things. And it is critical to have the right kind of discussion on where we stand. But when it comes to civil rights and our commitment to diversity, when it comes to our heroes - President John F Kennedy and Dr [Martin Luther] King - Senator Obama and I are on the same side. And so in that spirit, let's come together."

The controversy erupted after Mrs Clinton said talked in an interview eight days ago about Mr Obama's repeated comparisons of himself to President Kennedy and Martin Luther King, the assassinated civil rights icon - whose birthday is being celebrated across America today.

Mrs Clinton, trying to make a point about presidential leadership and the difference between words and deeds, said: "Dr King's dream began to be realised when President Lyndon Johnson passed the Civil Rights Act of 1964."

Some black leaders said they believed Mrs Clinton was diminishing Dr King's legacy. Mr Obama himself criticised Mrs Clinton for the remarks.

The dispute grew rapidly uglier, with Mrs Clinton and Mr Obama both accusing each other of injecting race into their contest to inflict damage. Mrs Clinton said the Obama campaign had "deliberately distorted" her remarks. Mr Obama called that claim "ludicrous".

The dispute has pitted civil rights leaders against each other in South Carolina, where half the Democratic electorate are African American and where the party holds its first primary in a southern state next weekend.

The fact that the subject of race, the issue burnt most deeply into the American psyche, has erupted in the campaign has dismayed many Democrats, who fear it is damaging both candidates in the run-up to the general election.

To underscore such sensitivities, a 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.

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