National MP Todd Muller will challenge for the National Party leadership on Friday, saying the party will not win the election with Simon Bridges at the helm.
Simon Bridges today summoned the party's MPs back to Parliament for an emergency caucus meeting at midday on Friday.
The phones have been running hot since a poll this week put the opposition party at a disastrous 30 percent.
Muller advised Bridges and the National Party president Peter Goodfellow of the challenge this afternoon and sent an email to his caucus colleagues shortly after 4pm, signalling his intent.
Bridges earlier said he was confident he had the numbers to win a vote.
It's still not clear where the votes would fall if there is a direct challenge, with one well placed MP - a Bridges supporter - telling RNZ it was a "line call".
Bridges said this morning a vote to test his leadership and that of his deputy Paula Bennett would be held by next Tuesday "at the latest".
Bridges said he was confident he and Bennett would survive a vote but he wanted to resolve the matter quickly so the party could "get back to what matters".
In his email, Muller said given the "enormous" economic challenge ahead, it was "essential" National win the election in September.
"Labour has failed to deliver on every measure it has set for itself in government. This will not change and the consequences of it being re-elected at this time will be catastrophic for two generations," he wrote.
"Our communities and our economy are at stake. It is essential National wins this election.
"I share the view of the majority of my colleagues that this is not possible under the current leadership."
Senior MP and former leadership contender Judith Collins said she would not be mounting a challenge.
"I've got no intention of being involved in it, any of that at the moment - no intention at all."
She declined to say who she would support in any leadership vote.
The numbers are still being done and while some MPs are privately backing a Muller-Kaye leadership team, many are still making up their minds.
Some MPs have questioned Bridges' strategy of announcing the upcoming vote during his round of media interviews this morning, but Collins would only say that "was entirely up to him".
When asked whether Bridges and Bennett would survive a vote of no confidence, Collins said that was up to the caucus, adding she "wasn't doing the numbers for herself or anyone else".
She said disunity within political parties in an election year was "terrible in terms of the people and the polls" and she urged the caucus to resolve this as soon as possible, then "come together in unity".
Mark Mitchell ran in the 2018 leadership contest, but said today he fully backed Bridges and would not be making a challenge.
"From day one I've pledged my loyalty to Simon and he retains that as leader," he said.
"Obviously there have been some caucus colleagues that have been active. We'll be going into caucus next Tuesday and whatever happens in caucus, we will emerge as a united team."
He would make no comment about Muller as a potential leader, or how things might play out in the event of a caucus vote.
Former National MP Jami-Lee Ross, who is now an independent, said he believed Simon Bridges would survive the challenge.
"I reckon you'll probably see the three camps - Mitchell, Collins, Bridges - come together and they'll be able to defeat Todd Muller.
"The difference with Jacinda and the way it worked for Labour was because their caucus could unite behind her. The National caucus is divided, they're not going to be able to bring it together in four months time and handing it over to someone who the public just don't know? I don't think it's gonna work."
"Todd Muller's got nothing to lose ... but why would you change from someone who the public deeply dislikes to someone who they just don't know?"
"As hard as it is for me to say, Simon Bridges is probably the one they have to stick with."