Former vice-presidential candidate and Hillary Clinton fundraiser Geraldine Ferraro sparked the latest storm in the Democratic presidential race today when she raised questions about Barack Obama's race, saying he has only been successful because he is a black man.

Obama's senior strategist David Axelrod called on the Clinton organisation to sack Ferraro from her position, saying Clinton should better "police the tone of this campaign".

"Leadership in campaigns comes from the very top," he said. "The signals that have been sent from the Clinton campaign have been really unfortunate."

Ferraro, who ran for vice-president on Walter Mondale's ill-fated 1984 ticket, told a California newspaper, "If Obama was a white man, he would not be in this position". She continued, "And if he was a woman ... he would not be in this position. He happens to be very lucky to be who he is. And the country is caught up in the concept". Ferraro implied that a woman of any racial background would face similar problems as Clinton has.

The Obama camp immediately jumped on the comments, demanding the Clinton campaign repudiate the remarks. According to CNN Obama's foreign policy adviser, Susan Rice, said: "I think if Senator Clinton is serious about putting an end to statements that have racial implications … then she ought to repudiate this comment."

The furore erupted as voters in Mississippi lined up at the polls to cast primary votes Tuesday. Mississippi, which will award 33 delegates based on Tuesday's vote, is predicted to go Obama's way. The Illinois senator has won a number of southern states, notably excepting Clinton's former home, Arkansas, where her husband Bill was governor.

The latest row in the increasingly heated and contest comes after Obama's foreign policy adviser, Samantha Power, was forced to resign after calling Clinton a "monster" in an interview with The Scotsman.

Obama's race has crept into the national dialogue despite the Illinois senator's efforts to portray himself as post-racial. As the two camps battled earlier this year, former president Bill Clinton compared Obama to Jesse Jackson, a Chicago minister, civil rights leader and former presidential candidate. The move was deemed an effort to belittle Obama as a "black" candidate who wouldn't appeal to white voters.

More recently, Pennsylvania governor Ed Rendell, an outspoken Clinton

backer, said conservative in his state, who go to the polls April 22,

weren't ready to elect a black candidate.

Joseph DiSarro, a political scientist at Washington & Jefferson College in Washington, Pennsylvania, predicted Rendell's comments would backfire by angering white voters who are not prejudiced.

"That's going to cost Hillary votes," DiSarro said. "There are going to be a lot of white voters who will be offended by that."

Obama's ethnic background - his father was Kenyan, and his middle name is Hussein - has become fodder for his opponents, mostly on the right. Initially, slurs accusing Obama of being a secret Muslim or a Manchurian candidate of sorts were found largely to the Internet.

But this week Iowa congressman Steve King, a Republican, said if Obama wins the White House, "the radical Islamists, the al-Qaida and the radical Islamist and their supporters will be dancing in the streets in greater numbers than they did on September 11th, because they will declare victory in this war on terror".

Although black voters have turned out en masse to vote for Obama and played a large roll in his southern victories, Obama's skin colour has not hindered him in mostly white states. On Saturday, for instance, he won Wyoming, which is 95% white, by 23 points.

Meanwhile, House speaker Nancy Pelosi, a California Democrat, dismissed the notion of a joint Clinton-Obama ticket as "impossible" – no matter who ends up winning the nomination.

In what appears to be a barb aimed at the Clinton camp, Pelosi, who has not publicly endorsed either candidate, told CNN: "I think that the Clinton administration has fairly ruled that out by proclaiming that Senator McCain would be a better commander-in-chief than Obama."

She explained her blunt assessment that a joint ticket would be impossible, by saying: "I wanted to be sure I didn't leave any ambiguity."

