Simon Bridges has criticised the Greens and New Zealand First for "voting on party lines" on euthanasia, rather than having individual MPs vote according to their conscience.
Bridges, leader of the National Party, said while he doesn't agree with legalising euthanasia, MPs shouldn't be forced to vote on party lines, when the End of Life Choice Bill has its second reading this week.
"I am really clear on the National MPs - they can go according to their views," Bridges told Magic Talk on Monday.
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He said the National Party and ACT Party - the latter whose leader David Seymour introduced the proposed law - are the "only ones who are exercising our true consciences".
"It's supposed to be a conscience vote, yet the Greens and New Zealand First are essentially voting on party lines, and New Zealand First wants to take it to a referendum."
Labour MPs will also be voting according to their conscience. Prime Minister Jacinda Ardern - who supports euthanasia - has said: "I see my view as a personal one and will leave the Labour caucus to make their own decisions."
The Greens said last month they'd support Seymour's Bill through its second reading, but on the condition it was amended before its third reading to only support assisted dying for the terminally ill.
New Zealand First is also expected to support the Bill's second reading, but only if an amendment is added before its third reading for a binding referendum on euthanasia at the 2020 election.
So just because the two parties have indicated they'll support the Bill's second reading, whether their support continues will depend on those specific amendments being passed.
Bridges slammed New Zealand First's call for a euthanasia referendum. He said it could complicate the 2020 election, with Kiwis already being asked to vote in a referendum about legalising recreational cannabis.
"I think if we get to the point where we're having a number of referendums it becomes very distracting from choosing a government to lead this country into the future."
Seymour has promised to amend his Euthanasia Bill if it passes its second reading, limiting it to those who have a "terminal illness that is likely to end the person's life within six months".
He said the Bill would also be changed so that age, disability and mental illness couldn't be considered viable reasons for euthanasia to be granted.
The Bill passed its first reading 76 votes to 44. But while that gives the impression it had wide support, Bridges said many MPs just wanted the Bill to get to select committee.
"The first reading you see these things go reasonably comfortable, because there's a lot of people who say, 'Well, let's have the argument and discuss this'," he told Magic Talk.
As for how he thinks the second reading will go, Bridges said: "My sense of it, it'll be incredibly close - it'll either just go through, or just not."
Only minor changes were made to the Bill after its first reading. The Justice Select Committee left the substantive amendments to be debated in Parliament.
Those minor changes to the Bill are what MPs will be voting on this week. It's the third reading to watch out for - when MPs vote on the Bill with its major amendments.
But even if the Bill passes its second reading, there's a chance it could be bogged down by the sheer volume of the proposed amendments.
For instance, National MP Maggie Barry - who has strongly opposed legalising euthanasia - has proposed about 120 amendments that could push the Bill's passing into next year.
The Bill has faced fierce opposition from faith-based organisations, to terminally ill patients and medical professionals. There are fears elderly people could be coerced into using it by family pressure.
But the Bill has also received widespread support. A Newshub poll last year found that 71 percent of New Zealanders were in favour of it.
Bridges said before MPs vote on Seymour's euthanasia Bill, they should "read about it, think about it, and then make a decision".