Battle lines drawn as McGuinness blows race wide open
That prompted speculation on what Independent TDs would support him, though there was little doubt he would easily get the required number.
Labour said it would be "interesting to see who they turn to for the three additional names required".
Fine Gael sources said Mr McGuinness's arrival would have an impact but it would also "harden the vote up" for other candidates in the field.
Fianna Fail sources said his presence would not change its decision not to run a candidate.
In America last night, Mr McGuinness said he was very honoured to have been asked to stand for the Presidency.
If selected at the ard comhairle tomorrow he intended to stand "on a broad progressive platform" and to build on his work in the peace process.
"I hope my campaign will give citizens the opportunity to make a stand for a new Ireland," he added.
But there were questions over whether Mr McGuinness's record in the peace process would translate into enough support for him to be elected.
"Will the Irish people want someone who conducted a murder campaign in the Aras?" a source said.
A number of political sources said there would be an element in the election of 'Anybody But Sinn Fein'.
The Northern deputy first minister will temporarily stand down from the power-sharing government for the campaign.
His entry into the race will put the focus back on the former IRA chief-of-staff's terrorist past. In particular, the President is the supreme commander of the Defence Forces. Yet six years ago, the government accused him of still being a member of the IRA Army Council -- a charge he denied.
Senior Sinn Fein figures admitted last night that Mr McGuinness would face personal questions during the campaign, similar to all candidates.
While his role in the peace process will be highlighted the Mid-Ulster MEP will again face a range of questions about his IRA past:
•The positions he held at the top of the organisation.
•When he ceased to be a member.
•His activities in Derry on Bloody Sunday.
•His knowledge of the Enniskillen atrocity, which killed 11 civilians.
•His relationship with the chief suspect in the Claudy bombing, which killed nine people.
Mr McGuinness has claimed he ceased being a member of the IRA 40 years ago.