Deputy Labor leader Tanya Plibersek says the opposition could push Malcolm Turnbull to breaking point over action on same-sex marriage, even if it costs him the prime ministership.
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With the government's planned legislation for a plebiscite on the issue set to be killed off in the Senate, Ms Plibersek said gay and lesbian families and advocates had told Labor they were prepared to wait in order to avoid a divisive public debate on the issue.
Ms Plibersek said Labor accepted the full moral and political consequences of the delay and called on Coalition MPs to back Mr Turnbull's own previous position of support for a free vote in Parliament on gay marriage.
The comments come as conservative backbenchers say they will abandon Mr Turnbull if he moves to allow a free vote, an apparent breach of the secret Liberal-Nationals Coalition agreement.
"It's not our job to save his job," Ms Plibersek told Sky News.
"It's our job to legislate, that's our day job, that's what we're paid to do. If only [Mr Turnbull] had some real power like being prime minister or something, wouldn't that be fantastic?"
National MPs George Christensen and Andrew Broad said last week that they wouldn't allow any move away from the government's policy of a plebiscite, prompting Ms Plibersek to label government MPs "a rabble".
"They are going to roll a Prime Minister because he wants to do what is in his heart.
"He's said before he supports a free vote in the Parliament. He should be allowed as Prime Minister to do what his heart and his head tell him is the right thing.
"They should back their Prime Minister on an issue of principle for him. They should back him," she said.
Last week Mr Turnbull refused to say if he would advocate a free vote once Labor blocks the plebiscite in the Senate, joined by the Greens and the Nick Xenophon bloc. The proposed legislation is still being considered by the House of Representatives.
Cabinet Secretary Arthur Sinodinos called on Labor to let the government keep its promises.
"That policy was enunciated or developed as a result of a marathon meeting of the party when Tony Abbott was prime minister and the agreement was there would be a plebiscite put to the election.
"The plan B, C, D and E is to keep persisting with getting this through the Senate," he said.
Speaking on ABC TV, Labor foreign affairs spokeswoman Penny Wong said Mr Turnbull's position was unsustainable.
"What I'd say to him and to the other moderates: have the courage of your convictions," she said.
"I have said publicly if there is a Liberal senator who wants to co-sponsor a bill with me, I'm open to it. I'm up for that."
