A bill to make Tasmania the first jurisdiction to legalise same-sex marriage appears headed for defeat in the state parliament's upper house, the Legislative Council.
Key undeclared MLCs in the 15-member chamber are coming out against the legislation ahead of a vote later today.
Among them are independent Hobart MLC Adriana Taylor, who said its constitutionality was unclear and that she was not persuaded by the decision of other countries to approve gay marriage.
"Because others are doing it is not a good enough reason for me," Ms Taylor said.
Another MLC, Liberal Vanessa Goodwin, said she had legal and constitutional concerns, although she did not oppose the bill on moral or religious grounds.
Legal objections are also expected to weigh heavily with a third undeclared MLC yet to speak — Jim Wilkinson.
Support for the change came from Devonport independent Mike Gaffney, who said he was unconcerned by the prospect of any constitutional challenge.
"Gays, when finding a partner, just want to ask the question: 'Will you marry me?' not 'Will you sign a deed of relationship?' "
Premier Lara Giddings made a last-ditch bid to urge the reform using Twitter. "Social norms change," she tweeted. "Once interracial marriages were condemned. Single mothers had babies forcibly adopted. People thought they were right.
"Today our Legislative Council have an opportunity to move our state forward and not be on the wrong side of history."