Republican presidential candidate Michele Bachmann insisted she would push on with her campaign in spite of having suffered two major setbacks on Thursday.

Her Iowa state campaign chairman, state senator Kent Sorenson, suddenly defected to support Ron Paul. Hours later, she apparently fired a second senior staffer for appearing to contradict her.

Bachmann alleged that Sorenson, who had served as her state campaign chairman for nearly a year, had jumped to Paul's campaign in exchange for cash payments.

Sorensen strenuously denied the allegations, saying he had decided to switch his support to Paul because the campaign had reached "a turning point".

He accused Bachmann of not telling the truth about his departure: "Sadly, the values I most appreciated in Congresswoman Bachmann appear to have gone out the window."

The row comes with only four full days of campaigning left before the Iowa caucuses. Polls on Thursday established Mitt Romney as frontrunner, pursued closely by Ron Paul and Newt Gingrich, with a late surge by Rick Santorum.

Bachmann, who has invested her hopes of the presidency on winning Iowa, has failed to make much of an impression on the polls. If she fails to make it into the top four in Tuesday's caucuses, she may quit the race.

Speaking in the small town of Nevada, Iowa, at the end of bus tour of the state's 99 counties, she dismissed the polls, insisted she could still win and portrayed herself as the Iron Lady, who would lead the US out of socialism to prosperity, just as she said Margaret Thatcher had done for Britain.

But she has lost the sense of confidence she displayed when, in what now looks to have been the high point of her campaign, she won the Ames straw poll in Iowa in the summer. The crowds at her campaign stops were sparse, and some of those who turned up said they planned to vote for other candidates or had not made up their minds.

Her campaign was being overshadowed by the defection of Sorenson. On Wednesday night, in a surprise development, state senator Sorenson attended a campaign event with Bachmann at the Pizza Ranch in Indianola. But hours later, he drove to a Paul rally to announce he was switching his support because of the way the Republican party was ganging up against him.

Bachman retaliated by claiming Sorenson, who had been with her from the early days of her campaign, had been bought.

"Kent Sorenson personally told me he was offered a large sum of money to go to work for the Paul campaign," Bachmann said.

That prompted her Iowa political director, Wes Enos, to put out a statement on Thursday morning, insisting money was not involved. "I cannot, in good conscience, watch a good man like Ken Sorenson be attacked as a 'sell-out'," Enos said.

Bachmann's press spokeswoman, Alice Stewart, said Enos "is no longer with us," apparently sacked.

Speaking in Nevada, with a group of supporters behind her, Bachmann said: "This is all about money." She said Sorenson had told her and others, including one prepared to testify under oath, he had been offered money to defect.

Sorenson, in a statement issued by the Paul campaign Thursday night, said he was saddened by Bachmann's response.

"As for the ridiculous allegations that Congresswoman Bachmann and her surrogates have made, I was never offered money from the Ron Paul campaign or anyone associated with them and certainly would never accept any. Financial reports come out in just days which will prove what I'm saying is true," Sorenson said.

"Even Congresswoman Bachmann's political director issued a statement defending my character. Since then, he's been fired by the Bachmann campaign for daring to tell the truth."