The man once tipped to be Ireland's first openly gay President said he did not regret seeking clemency for his former partner convicted of statutory rape as he quit the race over the controversy.
Senator David Norris said he was motivated by love and concern in writing the letter 14 years ago on behalf of Ezra Nawi, who had sex with a 15-year-old Palestinian boy.
As members of his campaign team quit and key backers withdrew their support, an embattled Mr Norris said he had made a human error in helping someone he loved.
"The fall-out from his (Ezra Nawi's) disgraceful behaviour has now spread to me and is in danger of contaminating others close to me both in my political and personal life," Mr Norris said.
"It is essential that I act decisively now to halt this negative process.
"I do not regret supporting and seeking clemency for a friend, but I do regret giving the impression that I did not have sufficient compassion for the victim of Ezra's crime."
The veteran Senator, an academic and renowned James Joyce scholar, did not take questions as he spoke to media outside his north Dublin Georgian home, but claimed he had been wrong.
"I accept more than a decade and a half later when I have now reviewed the issue, and am not emotionally involved, when I am not afraid that Ezra might take his own life, I see that I was wrong," Mr Norris said.
Prior to the revelations, Mr Norris had secured the support of 15 TDs and Senators to stand as an independent candidate in the race to succeed President Mary McAleese.
He needed the support of just five more parliamentarians to be declared a formal candidate, but three backers - Independent TDs Finian McGrath, John Halligan and Thomas Pringle - withdrew their support.