According to US media, the request to Mrs Palin - who Senator McCain later regretted selecting - was sent through a family intermediary.

A Palin "family source" told NBC News that "out of respect" to the family, "we have nothing to add at this point".

Mrs Palin was unveiled as McCain's running mate exactly 10 years ago.

"The Palin family will always cherish their friendship with the McCains and hold those memories dear," the family representative told a reporter on Wednesday.

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