FIANNA Fáil was last night propelled into a bitterly divisive leadership contest with the party's two main heavyweights, Micheál Martin and finance minister Brian Lenihan, both declaring their candidacy to become its eighth leader.

Another cabinet minister, éamon Ó Cuív – grandson of party founder Eamon de Valera – also entered the race.

The divisive contest comes after Brian Cowen's decision to resign as Fianna Fáil leader yesterday afternoon when it became apparent that his botched attempt to reshuffle his ministers had left him without allies either in cabinet or on the party backbenches.

However, Fianna Fáil hopes Cowen will be able to stay on as a caretaker taoiseach up to the 11 March polling day will hang on a crucial meeting of the Green Party parliamentary party this morning.

The Green Party's Dáil support will be required if the government is to win two confidence motions and head off an immediate general election. Fine Gael yesterday signalled it would put down a motion of no confidence in Brian Cowen as taoiseach when the Dáil resumes this week. This comes on top of Labour's no confidence motion in the government.

While the Green Party has consistently stated it would stay in government long enough to ensure the passing of the finance bill, TDs and senators will be reluctant to vote confidence in either Cowen or the government. Informed sources warned against predicting how this morning's meeting would go.

Fianna Fáil will vote on its new leader on Wednesday afternoon, but the campaign of former foreign affairs minister Micheál Martin received an early boost yesterday afternoon when 13 TDs – just over one third of the amount he will need – immediately came out to endorse his leadership. The list includes four TDs – Michael Mulcahy, Chris Andrews, Sean Haughey and Sean Ardagh – from Lenihan's Dublin heartland.

Confirming his candidature to the Sunday Tribune last night, Lenihan noted the public declarations for Martin but said the party had been "bounced into these matters in the past" and urged TDs to "sit back and reflect" rather than making instant decisions.

He said it was important that TDs hear what the different candidates had to say and added pointedly that he was "not encouraging public declarations on the day the Taoiseach had resigned as Fianna Fáil leader".

Martin said he "wouldn't take anything for granted and I will have to ask every single member of the parliamentary party for their support.

"This is the most important general election campaign in the history of the country. It is extremely important that we have a strong and focused debate on policy... and that is where our focus will be."

The contest could yet become a four-way race with Martin, Lenihan and Ó Cuív possibly being joined by tourism minister Mary Hanafin, who was said to be "taking soundings from backbenchers" last night.

But TDs believe it will be a de facto two-way battle between Martin and Lenihan, with the former holding the early advantage and boosted in no small way by his decision to oppose Cowen in last week's confidence motion.

Cowen took the decision to resign yesterday after returning home to Offaly on Friday night. He told a press conference yesterday in Dublin's Merrion Hotel that he had taken his decision "on my own counsel", following discussions with his family.

He said he had not discussed the matter with any senior party figures on Friday or Saturday, which could be taken as further evidence of how isolated Cowen had become within his own party.

It is understood Cowen's closest allies were being contacted by middle-ground TDs after Thursday insisting that he could not stay on as leader. There was speculation that senior figures close to him would tell him it was time to go. There was also a critical intervention by former tánaiste Ray MacSharry on Friday afternoon when he commented that the leader had responsibilities to the party.

Addressing the press conference, a composed Cowen said he did not want the situation to continue whereby his leadership was deflecting attention from the real issues and policies. "The unity of the party is far more important than my position," he added. While he was not definitive on his future plans, he seemed to indicate that he would be contesting the upcoming general election.

However, the date of that election remains in the balance today. If the Greens do not vote confidence in Cowen or the government this week then there will be an immediate general election. Cowen's decision to resign as Fianna Fáil leader, but stay on as Taoiseach will increase the pressure on the Greens not to vote confidence in either Cowen or the government. However, the party has consistently stated it would "do the right thing by the country" and stay in government until the finance bill was passed.

The Greens are facing three possible scenarios when they meet this morning.

They can stay in government and vote with Fianna Fáil on the confidence motions; vote with the opposition on the confidence motions precipitating an immediate general election; or withdraw from government but still vote with Fianna Fáil on the motions to ensure the Finance Bill goes through. Party sources declined to be drawn on which of these outcomes was the most likely.