A prominent black Democratic congressman switched his support from Hillary Clinton to Barack Obama last night, the latest blow to the former First Lady’s presidential campaign.

John Lewis, a civil rights icon and one of America’s most influential black congressman, said that Mr Obama’s overwhelming victory in Georgia’s primary on Super Tuesday — and in his own Atlanta district — was the reason for his change of heart.

He added: “Something’s happening in America, something some of us did not see coming. Barack Obama has tapped into something that is extraordinary.”

Mr Lewis becomes the latest of several Democratic “super-delegates” to switch from Mrs Clinton to Mr Obama, adding to the sense of the Illinois senator’s powerful momentum before next Tuesday’s contests in Ohio and Texas. Mr Obama was also endorsed yesterday by his fellow senator Byron Dorgan, of North Dakota.

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Mrs Clinton’s chief strategist said that her campaign had been misunderstood as senior aides began openly contemplating the end of her White House bid.

After a tense, high-stakes televised debate against Mr Obama on Tuesday night in which Mrs Clinton failed to inflict enough damage to change the dynamic of their race, internal dissent intensified inside her campaign, much of it aimed at Mark Penn, the former First Lady’s top adviser. With the fate of her campaign hanging on next Tuesday’s contests in Texas and Ohio, the knives have started coming out for Mr Penn, a clear sign of a campaign now seriously contemplating defeat.

Mrs Clinton, however, after boarding a flight to Columbus, Ohio, dismissed speculation about whether she would quit if she lost next week’s contests. “I don’t think about it like that,” she said. “I’m doing everything to win. I’m working hard, my campaign is working hard and I feel good about these upcoming contests.”

She claimed to be still optimistic and that Tuesday night’s debate had helped her to “draw some good contrasts” with Mr Obama. Mrs Clinton bridled at suggestions that she had failed to deliver a knockout blow, saying such language applied to a “prize fight — not a debate”.

In contrast to Mr Obama, who since emerging as the front-runner for the nomination has become increasingly shy of the media, Mrs Clinton was so keen to brief the press that she was still speaking to reporters as her plane’s landing gear was lowered.

With next week’s showdowns in Texas and Ohio looming, Leon Panetta, Bill Clinton’s White House chief of staff and a family loyalist, launched a scathing attack on Mr Penn. He called him a “political pollster from the past” and said that the campaign had no plan to take on Mr Obama.

Mr Clinton said last week that his wife must win Texas and Ohio to keep her White House hopes alive. She enjoyed double-digit leads in both last month but Mr Obama has pulled slightly ahead in Texas and to within five points in Ohio. Her national campaign chairman, Terry McAuliffe, echoed those comments: “If we don’t \, then she has to make a decision on what she’s going to do.”

Despite Mr Obama’s momentum, Mrs Clinton cannot be written off, and victories next week would dramatically reshape the race. She later held an economic solutions discussion in Zanesville, Ohio, a steel-making town that has been particularly hard hit by the decline in manufacturing industry. Highlighting her economic plan which promises to create five million new jobs and fix the burgeoning mortgage foreclosure crisis, she said: “I’m the only candidate with a plan to address what is happening out there.”

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