Mr Brendan Howlin has committed himself to working to put Fianna Fáil out of power and to construct an alternative government "anchored in the value systems of Labour".
The first candidate to announce his intention to run for the party leadership, Mr Howlin yesterday signalled a move away from the electoral strategy pursued by Labour in the general election.
Then, the party left open the option of coalition with Fianna Fáil, while stating a preference for a government led by Fine Gael.
However, Mr Howlin yesterday said: "I believe that we need to get Fianna Fáil out of government. I believe that strongly now.
"There are sufficient forces in Irish public life that could forge an alternative."
His aim, he said, was "to put together an alternative that would be anchored in the value systems of Labour".
He said he was not seeking to position Labour as part of a specific alternative government formation, such as one with Fine Gael. "I will not mortgage Labour's future to anyone else," he told The Irish Times.
"My job would be to oppose a bad Fianna Fáil-led government whose mandate is based on a con. I'm not anti-Fianna Fáil, but pro-Labour," he added.
He agreed, however, that his position in relation to Fianna Fáil was different than that adopted by Labour during the general election campaign "because we are in a different political time".
Mr Howlin's declaration yesterday will be followed in the coming days by announcements from a number of other candidates for the party leadership and deputy leadership.
Nominations for both positions close tomorrow week, with a postal ballot of the party's 3,600 members due to take place during October.
Mr Pat Rabbitte, Mr Eamon Gilmore, Ms Róisín Shortall, Ms Joan Burton and Mr Willie Penrose are among those who may announce their candidacies for the leadership or deputy leadership in the coming days.
In a statement yesterday, Mr Howlin said he would forge new alliances, in recognition of the changed composition of the Opposition in the Dáil since the election.
These alliances would be "on the broad left in Ireland, with a wide range of community, environmental and other groups and with the broad mass of people, especially young people, who are alienated from politics by tainted public behaviour".
The job of the Labour Party leader would be to "lead the left in Irish politics".
While the values of the Labour Party had never changed, "we live in a changing world and we address a changing constituency".
"So we have to find new ways of communicating our values."
He said that while the parliamentary numbers suggested that the Government might last a five-year term, "the reality is its authority has already been eroded because its basic dishonesty has been found out".
Mr Howlin concluded: "It will not be long before dissent comes to the surface across a wide range of areas."