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.
Mr McGrath said the protection of children and the integrity of the President's office must come first.
Members of the Senator's presidential campaign team also quit over the controversy, which centred on a letter he sent in 1997, written on Seanad notepaper, to Israeli authorities seeking clemency for Mr Nawi who had been convicted of having sex with a 15-year-old Palestinian boy.
As recently as two weeks ago Mr Norris topped the polls to win the presidential election, securing a quarter of the public vote ahead of Fine Gael candidate Gay Mitchell and Labour's Michael D Higgins.
But during a 10-minute radio poll on RTE's Liveline programme yesterday, 10,844 listeners said Mr Norris should not proceed with his nomination, while 8,758 believed the Senator should still go forward in the race for President.
The Senator said his former partner's actions were terrible, but that his motivation to write the letter was out of love and concern.
"I was eager to support someone who had been very important in my life," he said.
"As a senator I have always been an active and vibrant voice for those vulnerable and marginalised in society. So it is very sad that in trying to help a person I loved dearly I made a human error."
It is the second controversy to engulf the Norris campaign, after comments made in 2002 about sexual activity between older and younger men and boys resurfaced earlier in the summer.
In the days that followed those revelations Mr Norris said his campaign was being sabotaged by the reemergence of the comments.
The Senator said he had now demonstrated that it was possible for a gay person to be seen as a viable candidate for the highest office in the state - a once unimaginable feat for a country steeped in Catholicism.
"I would have loved to have had the opportunity as president of Ireland to extend that to the service of the entire people but that is no longer possible," Mr Norris said.
Mr Norris said the way in which Presidential candidates are nominated must be urgently examined by the Government.
After delivering his six-minute statement, he walked up the steps of his house without taking questions, turned and waved to the cameras and supporters.
Independent TD Catherine Murphy, who had pledged support for Mr Norris, said his decision to step out of the race was the correct one.
"I think that the letter in question does demonstrate a serious error in judgement on Senator Norris' part and the honourable thing is to take a step back from seeking a nomination, which he has now done.," she said.
Businessman Sean Gallagher, who is also seeking nomination to stand for the presidency, said he felt for Mr Norris and his supporters.
"Today is obviously a difficult day for Senator David Norris and on a personal and a human level I feel for him, his friends and supporters," Mr Gallagher said.
"I would like to acknowledge the work he has done in campaigning for many important issues throughout the years."
Joe Costello, Labour's Presidential campaign director of elections, said it is right to recognise the contribution made to public life by Mr Norris.
"As a public representative who shares a constituency with David Norris, a man for whom I have considerable admiration, I regret that he will not have the opportunity of going before the people in the presidential election," Mr Costello said.
© Press Association