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Senior Republican Senator Chuck Hagel has voiced doubts about Sarah Palin's qualifications for the vice-presidency.
John McCain's running mate "doesn't have any foreign policy credentials", Mr Hagel told the Omaha World-Herald.
Mr Hagel was a prominent supporter of Mr McCain during his 2000 bid for the US presidency, but has declined to endorse either candidate this year.
He was opposed to the Iraq War, and recently joined Mr McCain's rival Barack Obama on a Middle East trip.
'Stop the nonsense'
"I think it's a stretch to, in any way, to say that she's got the experience to be president of the United States," Mr Hagel told the Omaha World-Herald newspaper.
And he was dismissive of the fact that Mrs Palin, the governor of Alaska, has made few trips abroad.
"You get a passport for the first time in your life last year? I mean, I don't know what you can say. You can't say anything."
Mr Hagel also criticised the McCain campaign for its suggestion that the proximity of Alaska to Russia gave Mrs Palin foreign policy experience.
"I think they ought to be just honest about it and stop the nonsense about, 'I look out my window and I see Russia and so therefore I know something about Russia'," he said.
"That kind of thing is insulting to the American people."
BBC North America editor Justin Webb says Mr Hagel's opinion of Mrs Palin will have an effect on independent voters.
A senior member of the Senate Foreign Relations Committee, Mr Hagel was a close ally of Mr McCain, but the two men parted company over the decision to go to war in Iraq.
Mr Hagel skipped this year's Republican National Convention in favour of a visit to Latin America.
Mr Hagel's decision to accompany Mr Obama this summer on a trip to Iraq and Afghanistan, as part of a US Congressional delegation, led to speculation that he would throw his support behind the Democratic nominee.
However, a spokesman for the Nebraska senator insisted in August that "Senator Hagel has no intention of getting involved in any of the campaigns and is not planning to endorse either candidate".
