Presidential front-runner Sean Gallagher has admitted asking a convicted criminal and fuel smuggler for 5,000 euro in a fund-raising drive for Fianna Fail.
After being hit by an onslaught of attacks over his political past, Sinn Fein warned they would ask the donor to personally explain the circumstances of the invite.
Mr Gallagher revealed the damaging approach after facing down a string of broadsides during a live RTE television debate.
The former Fianna Fail member claimed he was not aware of the donor's past three years ago when he asked for the cash in return for a meet and greet with then taoiseach Brian Cowen. But he insisted: "I've no recollection of getting a cheque from this guy. I don't know the man very well."
Mr Gallagher exposed the donor's past when challenged directly over the payment by Sinn Fein's Martin McGuinness on the television debate.
He had already faced a barrage of criticism from the other six presidential candidates over his political background and business deals, including a bookkeeping error over a lodged 89,000-euro cheque.
Mr Gallagher hit back claiming the donor had leased a constituency office to Sinn Fein on behalf of party president Gerry Adams ahead of the general election last February.
The final live debates involving the seven candidates for the Aras were marked by a string of broadsides intended to assassinate the former Fianna Fail man's character.
Mr McGuinness and Senator David Norris led the charge in exchanges focusing on Mr Gallagher's fund-raising and business deals. "There was something very rotten at the heart of the last administration. As far as I'm concerned Sean was part of that," the Sinn Fein candidate said.
As the row over political backgrounds deepened, islanders off Donegal cast their ballots although turnout was low. People registered to vote on islands off Mayo go to the polls on Tuesday, while off Galway polling stations open on Wednesday before islanders off Cork vote with the rest of the country on Thursday. All ooters go to the polls on Thursday.