Colm Keaveney has quit as chairman and member of the Labour Party.
The resignation is another blow to the junior coalition partners who last week lost TD Patrick Nulty.
Mr Keaveney, Galway East TD, had lost the party whip after refusing to support the budget late last year but his position as chairman had caused the party hierarchy a headache. He said he was opposed to some of the reforms in the new abortion legislation.
"I have found that the more I articulate the views of members, or try to facilitate a discussion of real Labour policy, I am seen as a problem, a difficulty, an inconvenience to those who believe they know more and understand more than the people they represent," he said.
"Unfortunately I can no longer go along with what is increasingly like a political charade. We promise one thing then do another and blame it on someone else. The members must accept what they are given and the leadership will tolerate no dissent."
Mr Keaveney accused the coalition of targeting cuts at those least able to defend themselves. Referring to a U-turn by Education Minister Ruairi Quinn over cuts to special needs and resource hours in schools, Mr Keaveney said it is no way to run a country. He said he was becoming more alienated the more he tried to promote Labour values.
Mr Keaveney rounded on Labour ministers and said: "Too many at the cabinet table are willing to trade what they held dear for one more hour in the sun. Politics can change but only if we have the desire to make it happen. Politicians must be brave and must genuinely believe in something more than their own career."
Labour has lost a string of representatives since it took its biggest electoral success in early 2011. Former junior health minister Roisin Shortall quit after clashing with Health Minister James Reilly over the designation of primary care centres in his constituency.
Mr Nulty was kicked out of the parliamentary party after voting against the coalition's first budget in December 2011. He had only been in the Dail six weeks after winning the seat left by the late former finance minister Brian Lenihan.
Nessa Childers MEP quit in April, claiming the party was hurting people. TDs Tommy Broughan and Willie Penrose also resigned.