Judy Rose, a campaign volunteer in Jones County, was forced to resign last week after the campaign learnt of the email she had forwarded on November 21.
Rose explained that she sent the email to other area Democrats "to show them how dirty politics was getting".
"Iowans know garbage when they see it and it has no place in this race"
Bill Burton, Obama spokesman
A copy of the email provided to The Associated Press on Sunday read: "Let us all remain alert concerning Obama's expected presidential candidacy".
"Please forward to everyone you know. The Muslims have said they plan on destroying the US from the inside out, what better way to start than at the highest level."
Obama, a member of the United Church of Christ, has denied ever being a Muslim but rumours are circulating on the internet attempting to tie him to Islamic extremists.
Decrying the rumour as offensive and outrageous, the Clinton campaign earlier asked Linda Olsen, a volunteer in Iowa county, to resign after discovering she had forwarded a similar email to 11 people without any comment early this month.
Mo Elleithee, a Clinton spokesman, said the campaign's position was "crystal clear".
"Our campaign does not tolerate this kind of activity or campaigning. As soon as it came to our attention, we asked this individual to step down," he said, adding that all campaign leaders have been informed about the "zero tolerance policy for this type of activity".
Bill Burton, an Obama spokesman, said "Iowans know garbage when they see it and it has no place in this race".
On the campaign trail, Winfrey told a crowd gathered at a football stadium on Sunday that "it's your time to seize the opportunity to support a man who, as the Bible says, loves mercy and does justly".
|Recent polls show Clinton, left, still |
ahead of Obama in South Carolina [AP]
Campaigning in Iowa a day earlier, Winfrey said: "I'm sick of politics as usual. We need Barack Obama."
A recent AP-Pew Research poll had Clinton leading in South Carolina with 45 per cent of likely Democratic primary voters, followed by Obama's 31 per cent.