The President of Siptu, Jack O'Connor, says there is a poverty of ambition among parties on the left to get into government.
Siptu members will debate their political allegiance to the Labour party at their biennial conference in Cork this week.
General Secretary Joe O'Flynn has criticised the party saying there are things Labour has done in government which it should have fought against.
However, the Siptu President says there is a lack of ambition among parties on the left in general.
Mr O'Connor said: "I believe that if the people on the left believed that they could win, then they could provide a viable alternative.
"I think though that there is a poverty of ambition on the left and a refusal to believe that collectively we could win."