The Republican vice presidential candidate attracted criticism for accusing Mr Obama of "palling around with terrorists", citing his association with the sixties radical William Ayers.

The attacks provoked a near lynch mob atmosphere at her rallies, with supporters yelling "terrorist" and "kill him" until the McCain campaign ordered her to tone down the rhetoric.

But it has now emerged that her demagogic tone may have unintentionally encouraged white supremacists to go even further.

The Secret Service warned the Obama family in mid October that they had seen a dramatic increase in the number of threats against the Democratic candidate, coinciding with Mrs Palin's attacks.

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