Martin McGuinness has the same right as any other Irish citizen to stand for election as president when he formally obtains the signatures of 20 Oireachtas members required for nomination. But the right carries with it a corresponding duty.
In a presidential election, more than any other, a candidate needs an exceptional record. Accordingly, he or she must answer, truthfully and fully, the innumerable questions that will be asked about his or her current views and previous career.
To date, Senator David Norris is the only contender to have felt the full force of media and public pressure.
Others have struggled to explain themselves but attracted little criticism.
This will most certainly not be the case with Mr McGuinness. Every hesitation, every apparent contradiction will be picked up and analysed.
That does not apply merely to questioning about his role in the Provisional IRA, which may be forgiven but must not be forgotten.
All candidates have an obligation to outline what they intend to do in the event of reaching Aras an Uachtarain, and the outline should include some overall vision for the country.
The candidates who entered the field earlier have been less than successful in this regard.
But Mr McGuinness has begun his campaign with a soundbite which must take the prize for sheer meaninglessness.
What does the term "people's president" mean? In a word, nothing.
Voters will also be conscious that the entry into the race of this outstanding Sinn Fein figure has serious implications -- never envisaged by the authors of the Constitution -- for party politics in the Republic.
Sinn Fein's ambition is to displace Fianna Fail from its present, humble enough position as the third party in the State.
In this endeavour, it has played its hand cleverly since the general election, except for one brief period when it showed unwonted clumsiness in its attitude to Queen Elizabeth's visit.
It certainly saw an opportunity in Fianna Fail's decision not to contest the presidential election.
The decision has turned out to be a gift for Sinn Fein. It has led to a major crisis in Fianna Fail, with some members of the parliamentary party in almost open rebellion against Micheal Martin's leadership.
There are now fears for the party's survival as a force in Irish politics.
Tomorrow is the deadline for Mr Martin to resolve these issues.
He finds himself in a deeply unhappy position, in which a desirable or even a plausible solution seems unattainable.
He must find his strength among level-headed deputies and senators who cleave to the party's old virtues of loyalty and discipline -- and know the value of political stability.