Liberal senator Dean Smith says he might cross the floor and vote against the government's marriage equality plebiscite in the senate, where the numbers are already on a knife edge.
Smith told the ABC's Radio National Drive program on Friday that he has significant concerns about the plebiscite and has not yet decided if he will vote in favour of it.
"I'll be waiting to see what the plebiscite detail is. That is the responsible decision," he said. "It's a very live issue for me."
Losing the support of a government senator would be a significant blow for prime minister Malcolm Turnbull, who already faces an embarrassing loss on the plebiscite bill in the senate.
The government has currently secured 37 votes for the legislation (or 36 without Smith), but needs 39 for it to pass.
If Labor can be convinced to vote for the bill, it will pass. However, if Turnbull is left trying to scratch together independents and renegade Greens for the last two votes, Smith's vote could sound the death knell for the controversial policy.
Smith cited the $160 million cost of the vote and the precedent it may set as his two central concerns.
"If we do have a plebiscite on one issue, there's a very powerful argument we should do it on the next controversial issue," he said.
"Do people think we should have plebiscites on family law... or on euthanasia issues? Some people in the parliament believe we should be having plebiscites on military deployment."
Smith said there were a number of issues to be canvassed before the full detail of the plebiscite would be revealed, including whether each campaign will receive public funding, and the structure of the vote.
On Friday, shadow attorney general Mark Dreyfus ramped up Labor's calls for the plebiscite to be binding, saying the government should introduce a bill that automatically legalises same-sex marriage if the plebiscite returns a "yes" vote.
"We think that is an absolute minimum requirement. I’m not saying it would be sufficient to win Labor’s support for a plebiscite," Dreyfus said.
However, Smith said he cannot support a binding plebiscite as he believes Liberals should not be bound in any vote.
"I'm one of those people who believes Liberals are ultimately given an opportunity, always, to exercise their conscience."
Lane Sainty is a reporter for BuzzFeed News and is based in Sydney, Australia.
Contact Lane Sainty at firstname.lastname@example.org.
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