Independent MP Peter Dunne could hold the crucial vote that decides whether a bill extending the powers of New Zealand's electronic spy agency becomes law.
But Mr Dunne, who was stripped of his parliamentary party funding on Tuesday after the Electoral Commission de-registered United Future last week, opposes the legislation in its current form.
Mr Dunne says the Government Communications Security Bureau (GCSB) should not be able to spy on New Zealanders, even on behalf of police or the Security Intelligence Service.
Prime Minister John Key has admitted he depends on New Zealand First or Peter Dunne for support and said on Tuesday that Mr Dunne understands the importance of the spy agency's work.
Mr Key is confident that Mr Dunne will vote for the Government Communications Security Bureau Amendment Bill. However, Peter Dunne has refused to respond to his comments on Tuesday.
New Zealand First leader Winston Peters said Mr Key needs to forge a consensus to pass the legislation, but there has been no sign that he is genuinely willing to engage.
In a submission on the bill on Monday, the New Zealand Law Society told MPs there is no clear justification for the Government spying on its citizens or residents.
Labour Party David Shearer said he shares concerns expressed in the submission and on Tuesday accused John Key of botching the handling of the legislation, saying it's his fault that support for the bill is hanging in the balance.
"He personally has bungled this right from day one - whether it be (Kim) Dotcom or the illegal spying, the appointment of (GCSB director) Ian Fletcher - all of this is around John Key's neck and he wants to push through a piece of legislation and say it's all okay - well, it's not."
Mr Shearer said Labour would only support legislation which emerges from a full review of the country's intelligence services.
In Parliament on Tuesday, Mr Key accused Labour of being prepared to put New Zealand's security at risk for political reasons and criticised David Shearer.
"I say to that member if he doesn't want to keep New Zealanders safe, if he doesn't want to ensure that we have better information about the security of New Zealanders, well, keep going the way you are son - but you won't become Prime Minister doing that."
Mr Key also rejected Labour's call for a review of spy agencies.