The campaigns of Barack Obama and Hillary Clinton are pointing fingers at each other over who is to blame for bringing the spectre of racial politics into an election where Democrats are increasingly dividing along ethnic lines.

Mr Obama’s grip on the black vote has strengthened so much over successive contests that he won 92 per cent support from African-Americans in Mississippi on Tuesday — a primary in which Mrs Clinton was backed by almost three quarters of whites.

Yesterday Mr Obama said that a racial dimension was “unfortunate”, if not “entirely unexpected”. Although he cites his success in attracting white support in Wisconsin and Virginia, Mrs Clinton has led among such voters in 23 out of 30 contests.

Both sides accept that American society still carries deep scars from slavery and segregation. Mr Obama said that these would take time to heal, but claimed that his campaign held out the promise of a future “where these distinctions are less prominent”.

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

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Hours earlier, the former vice-presidential nominee Geraldine Ferraro had resigned from Mrs Clinton’s campaign over her remarks suggesting that Mr Obama had risen as quickly as he had because of the colour of his skin. “If Obama was a white man — or a woman of any colour — he would not be in this position,” she said.

Mr Obama has described her comment as “wrong-headed”, rather than racist. But he also says: “The Clinton campaign has talked more during the course of the last few months about what groups are supporting her and what groups are supporting me. That seems to track certain racial demographics.” David Axelrod, his chief strategist, has gone further, suggesting that an “insidious pattern” was being weaved by the Clinton campaign.

Ms Ferraro claims that it is the Obama campaign that has “played the race card”. She said: “What I find offensive is every time somebody says something about the Obama campaign you are accused of being racist.”

There is palpable anger within Mrs Clinton’s campaign at the way her support among black voters has evaporated. In Philadelphia this week, a rally in a largely black neighbourhood attracted an almost entirely white crowd. Bill Clinton, beloved as a president by African-Americans, has seen his negative ratings soar since allegedly making racially charged remarks during the South Carolina primary.

A prominent black pastor, the Rev Eugene Rivers, claimed yesterday that the election had become a “virtual race war” and that African-American voters may skip the November general election if Mrs Clinton is the nominee and Mr Obama “is told to sit at the back of the bus”.

Mrs Clinton’s camp claims that, for all Mr Obama’s post-racial appeal, he has deliberately stoked controversy before primaries in states such as South Carolina with large black populations. For instance, having previously accepted Mrs Clinton’s assurance that she had nothing to do with the distribution of a picture showing him wearing a turban and tribal outfit, Mr Obama used a speech in Mississippi this week to accuse the Clinton campaign of leaking the photograph “to make people afraid”.

Sean Wilentz, a history professor at Princeton University, is among the Clinton supporters who believe that her campaign has been slow to recognise how Mr Obama was galvanising support among black voters and white liberals. He accuses the Obama team of having “purposefully polluted the contest with a new strain of what historically has been the most toxic poison in American politics”.

Asked why so many whites were voting for Mrs Clinton in states such as Mississippi, he stressed that those who had stayed Democrats in the South are unlikely racists, but added: “If there is a racial element, the question is how did it happen and in whose interests was it for Hillary to lose her black following?”

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