Apsley MLC Tania Rattray has confirmed she will vote against same-sex marriage, unconvinced the majority of Tasmanians support the change.
Ms Rattray was one of four independent MLCs who had refused to declare their voting intentions prior to today and without her support, the legislation is doomed.
Even if all three of the remaining undeclared MLCs voted in favour, the result would be a tie, giving President Sue Smith, who is against the legislation, the deciding vote.
Ms Apsley acknowledged the tide was turning, but not enough to justify Tasmania going it alone.
``Tasmania should be cautious about diving head first into an area with a potentially expensive High Court constitutional challenge as a likely result.''
Murchison MLC Ruth Forrest opened the much-anticipated debate shortly before 5pm, speaking for an hour.
She read out moving accounts from gay people and parents of gay couples as she made the case for making Tasmania the first state to legalise gay marriage.
She appealed directly to her wavering colleagues, asking them to consider the question:
``If they had or have two or more children, one or more of whom are heterosexual, would you want each of your children to be treated the same way, able to express and receive love in the same way and with the same community and legal recognition as their sibling or siblings?''