Sean Gallagher is a former party member who had successfully distanced himself from the Fianna Fail brand throughout the election.
But McGuinness charged that he had accepted checks on their behalf up to three years ago despite Gallagher's pledges that he had not.
Gallagher has denied he took the check but the damage was done in terms of igniting a media frenzy.
He has claimed a Sinn Fein conspiracy to tear him down. They have claimed he was hiding his Fianna Fail past and misleading people about it.
Gallagher's cause was not helped by another moment in the final debate when a woman in the audience slated his explanation of how a check that was supposed to end up in his company's bank account was assigned elsewhere.
The clear underlying accusation was tax evasion.
So Gallagher was suddenly firefighting massive flames on two fronts.
Yet he is still a likely winner.
He held a 15 point lead in polls going into the final weekend over Michael D. Higgins in second place.
Whether that size of a lead could evaporate overnight remains the unknown question.
My guess is it won't-- but it may be a very close run thing.
Either way an extraordinary end to an incredible presidential campaign.