Geraldine Ferraro, the former Democratic vice-presidential candidate, resigned last night from Hillary Clinton’s campaign after declaring that Barack Obama was successful only because he is a black man.

Ms Ferraro’s comments, which emerged on Monday, were condemned by Mr Obama and bought fresh charges of “insidious” tactics against the Clinton campaign by the Illinois senator’s chief adviser.

Ms Ferraro initially blamed the Obama campaign for stoking the controversy, an accusation that Mr Obama denied yesterday.

In her resignation letter to Mrs Clinton, she said: “The Obama campaign is attacking me to hurt you. I won’t let that happen.” She quit as a member of the former First Lady’s finance committee.

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

• Analysis: Florida rerun on the cards after Obama's latest win

• Clinton ally sparks Obama race storm

• Obama sweeps to Mississippi victory

Multimedia

• Read our dedicated US elections blog

Ms Ferraro, the 1984 Democratic vice-presidential nominee, said of Mr Obama: “If Obama was a white man, he would not be in this position . . . He happens to be very lucky to be who he is.” Her resignation came on a day when Mrs Clinton demanded a rerun of the disputed primary contests in Florida and Michigan, part of a strategy to capture the Democratic nomination by belittling Mr Obama’s lead among elected delegates.

With only ten contests remaining in their battle, Mr Obama holds a near-insurmountable lead among elected delegates. It is an advantage that he increased on Tuesday night with his win in the Mississippi primary, where more than half of the electorate was black.

Neither candidate is likely to gain enough elected delegates to win the contest outright, however, and Mrs Clinton is locked in a struggle to convince uncommitted super-delegates that she has the greater claim to the nomination.

Central to diminishing the significance of Mr Obama’s elected delegate lead is agitating for a rerun in Michigan and Florida. They held primaries in January, and Mrs Clinton won both, but the results were nullified because the two states broke party rules by moving their election dates up the nominating calendar.

In Michigan Mr Obama was not even on the ballot and neither candidate campaigned in Florida.

The two states are big, key general election battlegrounds. Mrs Clinton is favoured to win both again if the contests were reheld.

If she also wins the next contest, in Pennsylvania on April 22, her aides believe that she has a powerful argument to make to super-delegates: that despite Mr Obama’s elected delegate lead, she has actually won many more big contests that will be crucial in the general election.

After an intensifying debate in recent days among the Democrat leadership and the two campaigns over what to do about Michigan and Florida, Mrs Clinton entered the fray: “In my view there are two options — honour the results or hold new primary elections.”

David Plouffe, Mr Obama’s campaign manager, called the demand “an increasingly desperate strategy for what they think can help them win the nomination”.

Democratic leaders want the issue resolved but are worried about a disputed vote in Florida deciding the contest, as in 2000.

Times Recommends

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