The Labour leader, Mr Ruairi Quinn, has criticised the Taoiseach, Mr Ahern, as a power-seeker "honed in the Haughey tradition".
He made the remark in a speech designed to reduce speculation that his party will consider coalition with Fianna Fail after the next general election.
At a party meeting in Cork last night, Mr Quinn also said that while he did not rule out a coalition with the Progressive Democrats on the basis of an agreed policy programme, he could see no circumstances in which the two parties could agree such a programme.
"Labour and the Progressive Democrats are not parties that can do business together," he said.
While criticising Mr Ahern's record as Taoiseach and his party's record in Coalition, Mr Quinn nevertheless left open the prospect of entering a coalition with Fianna Fail after the next election. "Labour's electoral strategy is its own business", he declared, saying this comment was aimed at Fianna Fail as well as "the more presumptuous members of Fine Gael".
Mr Quinn disputed Mr Ahern's claims, made in a speech in Kilkenny two weeks ago, that Fianna Fail was the most successful party at constructing and maintaining coalition governments.
Mr Quinn said the reality was very different: "Each and every attempt at coalition by Fianna Fail has collapsed in ignominy. And if the Progressive Democrats had not abandoned any sense of pride and purpose, this Government too would be over."
The Taoiseach's addition to his party's knowledge of coalition-building was "minimal and minimalist. Honed in the Haughey tradition, power for power's sake remains the order of the day."
Mr Quinn combined criticism of Mr Ahern's ability to run a coalition with a claim that he was allowing the gap between rich and poor to grow.
Mr Ahern had a "carefully cultivated populism", but he had "appointed the nation's most right-wing ideologue to be his Minister for Finance and gave him carte blanche to further increase the divide between the well off and the rest of society.
"Bertie Ahern, the so-called `social democrat', is an illusion."
He also rejected the regular speculation that a Fianna Fail/Labour coalition is likely because he has a good personal relationship with Mr Ahern.
"How are the public to know that it is simply not true? Civility with colleagues whose career has intertwined with mine should not be confused with political support or sympathy - none exists."
Mr Quinn said political debate in Ireland was often not just ideology-free, but policy-free. "How else can you explain a Taoiseach perceived as making overtures to one party which he describes as a high-tax, high-spend party while serving in Government with a party that wants to slash tax rates and which rejects the European social model lauded by the other?"