Tim Pawlenty, a close ally who was on Mr Romney's vice presidential shortlist, left his post to become the chief lobbyist for America's banking industry.

The news came during the Republican hopeful's worst campaign week so far and coincided with an attempt to restyle himself as a champion of the poor after a damaging video had seen him effectively disown half the American population as 'dependents'.

Mr Pawlenty, who had been widely expected to take a job in a Romney cabinet, hailed Mr Romney as "a truly good man and great leader", and stressed that he would be banned from party-political work in his new job.

However, he did not explain why he needed to leave the election campaign immediately. He is due to take up his new role on November 1 and is expected to be paid about $1.8 million (£1.1 million) a year.

"Tim Pawlenty jumps the sinking S.S. Romney," declared Vanity Fair magazine. Jeffrey Sachs, the economist and Columbia University professor, accused Mr Pawlenty of "bailing ship" and said his move "exposes the systemic corruption" between politics and big business...

Mr Pawlenty will become Wall Street's emissary to Washington less than a year after he said in an interview that as president, his "message to Wall Street" would be: "Get your snout out of the trough".