Sunderland say claims Paolo Di Canio has racist or fascist sympathies is insulting to both the "integrity of the club" and their new manager.
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The Italian was appointed boss of the Premier League club on Sunday following the dismissal of Martin O'Neill.
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Di Canio, 44, apologised for causing offence with statements of a political nature but said he had been misquoted.
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"It's been disappointing to read some of the
reaction," said Sunderland chief executive Margaret
Byrne.
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Ex-foreign secretary David Miliband resigned from the board of Sunderland because of Di Canio's "past political statements" but Byrne said Di Canio's appointment had been turned into a "political circus".
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"To accuse him now, as some have done, of being a racist or having fascist sympathies is insulting not only to him but to the integrity of this football club," added Byrne.
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"Anyone who has met Paolo and spoken with him personally, as we did in depth before making this appointment, will know that he is an honest man, a man of principle and a driven, determined and passionate individual.
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"It is disappointing that some people are trying to turn the appointment of a head coach into a political circus."
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In a statement released by the club, Di Canio said he was hurt by his portrayal in the media since becoming Sunderland manager.
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"Of course it hurts because people try to take your dignity and that is not fair," said Di Canio.
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"I don't have a problem with anyone. I don't know why I have to keep repeating my story, to be defending myself on something that doesn't belong to me every time I change clubs."