Labour MP Louisa Wall is hoping the Prime Minister's near-endorsement of her bill to legalise same-sex marriage could give it the legs to pass into law.
Ms Wall's marriage equality bill, pulled from the ballot last Thursday, was given a significant boost yesterday when Mr Key hinted he would extend his support beyond the first hurdle.
Mr Key has previously said he would support the bill at the first reading, but said yesterday his conscience vote was unlikely to change if the bill progressed.
"You'd have to go through all the merits of the argument then see what people put up, but my view has been that if two gay people want to get married, I can't see that it would undermine my marriage to Bronagh, I just don't understand how that logic applies," he said in a radio interview.
"Let's go and have all the debates, I haven't spent a long time thinking about it. I'm giving myself a bit of room but in principle, I'm not opposed to it."
Ms Wall said she was buoyed by the support of the National leader and coalition leaders Tariana Turia and Peter Dunne, particularly because all three had voted against civil unions in 2004.
"At the end of the day [Mr Key's] vote and my vote are worth the same. But ... I don't under-estimate the Prime Minister coming out really clearly and signalling to the rest of the country that we shouldn't be afraid of having this discussion."
A Herald survey of MPs found 53 of them planned to support the bill or were leaning towards supporting it. It needed 61 votes to pass.
Seven National MPs have said they would definitely back it at least to the committee stage, including Cabinet ministers Paula Bennett and Hekia Parata. Most of the Labour Party and all the Greens were backing it.
But there was still a significant amount of hesitancy from National MPs, 35 of whom had not made up their minds.
Mr Key said he didn't think discussion of the bill would have the same intensity as the homosexual law reforms in the 1980s.
But the public debate did become more heated yesterday, when creators of an online petition opposing the changes alleged their site had been hacked.
Lobby group Family First, which set up the site, earlier released a statement that said the bill would open the door to polygamy.
Labour MP Charles Chauvel, who helped draft the legislation, dismissed the "tiresome" criticism. "All this bill does is open up the definition to include couples of the same sex. If somebody wanted to come along and sponsor a polygamy bill or some such outlandish notion, then they have to do that explicitly, get the support of the public for it and make it happen.
"It's a far cry from the sort of thing that Louisa's proposing."By Isaac Davison @Isaac_Davison Email Isaac