SINN Féin has issued a legal letter to RTÉ calling on it to immediately reverse its decision to hold a two-way leaders' debate without party leader Mary Lou McDonald.
The party has argued in recent days that its opinion poll ratings mean that Ms McDonald should not be excluded from two-way leaders' debates involving Fine Gael's Leo Varadkar and Fianna Fáil's Micheál Martin.
The first of these debates takes place on Virgin Media Television tonight with Ms McDonald not taking part.
But RTÉ is also hosting a two-way debate between the party leaders on Tuesday, February 4th - just four days before polling.
Ms McDonald has said it is a "farce" that her party has been excluded from these debates.
In what will be seen as the first step to a possible court challenge, the party's solicitors Prospect Law have, on behalf of senior Sinn Féin TDs Pearse Doherty and Eoin Ó Broin, written to RTÉ's election steering group tonight.
The letter argues that the criteria established for holding the two-way leaders' debate does not take proper account of the make up of the 32nd Dáil including that the confidence and supply deal means "Sinn Féin has been the largest voice of opposition in the Dáil and therefore the criteria fail to have due regard to the current political reality and they do not reflect current electoral representation".
The three-page letter asks the steering group to reconsider its position and to provide an appropriate response to the legal letter by the close of business on Thursday.
The letter states: "Given the precedent of multi-party participation in leader's debates since the 2007 election, we consider that it cannot reasonably be contended that there is sufficiently compelling broadcasting justification for an action which demonstrably likely to unfairly influence voters in a manner which runs counter to the democratic principles which underpin our Constitution, which principles require to be respected by RTÉ in the decision making process."
Sinn Féin's solicitors say they strongly-dispute that RTÉ's criteria for holding the two-way debate are "fair, objective or impartial".
The letter says: "Our clients contend that the holding of a two-way leaders' debate in the manner proposed is not necessary to meet any legitimate end or objective and does not service any or any sufficiently weighty broadcasting or editorial objective."
It argues that framing the debate as "head to head" between the Fianna Fáil and Fine Gael is "likely to have an impact on the election result in a manner which is inimical to the constitutional value and protection afford to democracy".
RTÉ confirmed it had received the letter tonight and would respond in due course.
TAOISEACH Leo Varadkar vowed that he would participate in a three-way televised debate with Fianna Fáil leader Micheal Martin and Sinn Fein leader Mary Lou McDonald if a TV station sought such an event.