Fianna Fáil leader Micheál Martin has said he "sees no upside to voting No" in the upcoming fiscal treaty referendum.
He was speaking after the former deputy leader of his party, Éamon Ó Cuív, said he would campaign for a No vote.
The official campaigning period for the referendum on the fiscal treaty started today, when Minister for the Environment Phil Hogan signed the Referendum Order.
It fixed the date for the referendum for Thursday 31 May.
He said he had relinquished the deputy leadership of Fianna Fáil, a position that had been very important to him.
He said: "I think if you vote No, you run the risk of a more accelerated pace of austerity than we are currently experiencing, because you create an insecure and uncertain scenario in terms of our capacity to access funds."
Mr Martin said he also disagreed with Mr Ó Cuív's assertion that Sinn Féin would be Fianna Fáil's preferred future coalition partners.
He said there had been a lack of accountability and "a fundamental dishonesty" from Sinn Féin about the Provisional IRA campaign of violence from the mid-70s.
Mr Martin also said he disagreed with Sinn Féin's policies on the economy and Europe.
Taoiseach Enda Kenny said the forthcoming vote on the fiscal treaty would be more important than a General Election vote because the consequences would remain for a far longer time.
He was speaking in Galway at the announcement of 115 jobs at US multinational, Cisco, in Oranmore.
Mr Kenny said the decision by companies of the calibre of Cisco to invest in Ireland demonstrated the value of economic stability to Ireland.
He said it was important for these companies to know they are investing in countries where there is access to funds from Europe in the case of an emergency.
The Taoiseach said the countries that ratify the treaty will have access to ESM funding. If Ireland votes No, he said, it was a vote for uncertainty and further austerity.
IMF says report 'a misinterpretation'
The International Monetary Fund has said that a newspaper report that suggested the IMF could be an alternative source of loans to Ireland contained "a misinterpretation of the remarks of a spokesman".
The IMF said its current lending to support programmes in eurozone countries has been undertaken together with its Troika partners.
Speaking this morning, Labour Party leader Tánaiste Eamon Gilmore said the fiscal treaty would bring stability to the euro, to the country, and would allow Ireland to access emergency funding if required.
The Tánaiste also said it would undermine international confidence in Ireland if the treaty was rejected.
He said the Labour campaign would involve members going to door-to-door talking to people one-to-one.
Sinn Féin and the United Left Alliance will be campaigning against the treaty, saying it would introduce permanent austerity.
The Irish Congress of Trade Unions decided not to issue any recommendation to union members on how to vote.
The Civil Public and Services Union, Mandate, Unite and the Technical, Engineering and Electrical Union have all come out against the treaty, officially known as the Treaty on Stability, Coordination and Governance in the Economic and Monetary Union.
SIPTU has only offered support conditional on a Government stimulus package.
The business bodies IBEC and the SFA are urging a Yes vote, as is the Irish Farmers' Association.
The Referendum Commission will be sending out its guide to the referendum to two million households on 8 May.
Its website will become operational this week at http://referendum2012.ie/