Tony Abbott's former chief-of-staff is predicting Malcolm Turnbull won't be prime minister by Christmas.
Peta Credlin says gay marriage and climate change will split the Liberal Party and doom his leadership.
'I don't think that Malcolm Turnbull will be here at the end of the year,' she told Sky News on Wednesday night.
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Peta Credlin (pictured) says Malcolm Turnbull was likely to be gone by the end of 2017 as PM
Malcolm Turnbull (pictured) is losing support from right-wing Liberal MPs over gay marriage
Asked by host Paul Murray if Mr Turnbull would have to resign as prime minister, she said a postal vote on gay marriage was the only thing that could stop conservative Liberal MPs, including right-wing cabinet ministers, from moving against him.
'It's interesting that the postal vote matter that's pushed forward by the conservatives Mathias Cormann and in particular Peter Dutton, but for these two men Malcolm Turnbull would not be prime minister,' Ms Credlin said.
If Mr Turnbull lasts beyond September 12, he will surpass Mr Abbott's two years as prime minister.
She argued this would help to soothe anger among Liberal MPs, after the Senate rejected Mr Turnbull's election pledge for a plebiscite on changing the Marriage Act.
'There's a safety valve in all of this, and that safety valve is the postal vote,' she said.
She said Mr Turnbull should take his idea for a postal vote to cabinet on Monday, and explain the terms of it to his party room on