Labor Senator Doug Cameron says party rules that prevent MPs from speaking out or voting against the party line is akin to having a "political lobotomy".

The former union leader, who is the co-convenor in the national left of the Labor Party, says Labor is in danger of creating "zombie politicians".

And he says it is "absolutely crazy" for Labor to say it is a progressive party but refuse to support gay marriage.

His criticisms come as today's Nielsen poll in the Fairfax press showed Labor losing support to the Greens.

Senator Cameron says MPs should be able to voice their views beyond the caucus if they disagree with party policy.

"I still don't think it is sufficient just to have behind-closed-doors debates on key issues," he told The World Today.

"I think the party needs to have a voice on a range of issues with a range of different views.

"I've always taken the view that difference of opinion is not a weakness, it's a strength and I think that has been lost in the party.

"We don't want zombie politicians."

Prime Minister Julia Gillard has again today ruled out support for gay marriage by Labor but Senator Cameron says he strongly disagrees.

"I just think it is absolutely crazy for a progressive party," he said.

"I just don't understand it, and to be honest, you are not allowed to speak about this in public from where we are and I just think that is an example of where we need to change."

Despite the pressure from the left, Ms Gillard says the rules will not be changed and MPs have ample time to debate issues in caucus.

"I want our caucus to be a place of debate, but we come from a political party that believes we are strengthened by being members of a team and the way the team works is we have those discussions internally," she said.

Greens Senator Christine Milne says her party is picking up support from Labor because the ALP has moved to the right in recent years.

"It has tried to sit in the middle of the road and has got run over in a lot of those seats - that's the classic case because they are losing the progressive vote to the Greens," she said.

"Then they try to go back further to the centre to pick up the more conservative votes and the Coalition pulls them further to the right and so they lose more of those votes on the progressive side of politics."