National Party leader Simon Bridges says he won't be voting for a referendum as part of the voluntary euthanasia legislation.
The End of Life Choice Bill, sponsored by the Act Party leader David Seymour, will be back before Parliament tomorrow.
MPs will decide on a number of proposed changes, including putting it to a referendum, which is a condition of New Zealand First's support.
Mr Bridges said he would vote against that.
"I think it's for members of Parliament to make up their mind on these things, rather than kick them for touch to a referendum."
In September, National MPs put forward around 20 amendments to the bill. National's Māori, Pasifika, and ethnic MPs also tried to push for cultural considerations and the Treaty of Waitangi to be properly addressed by the bill.
Meanwhile, Prime Minister Jacinda Ardern said she could vote for a referendum if that's what it took to get the legislation across the line.
"If I assess that that's the only way that the bill could continue and survive, then that's something I will give consideration to. My preference, of course, would have been to allow just Parliament to make that decision."