National Party leader Simon Bridges has ruled out working with New Zealand First in any potential government after this year's election.
Bridges made the bold announcement to kick off the party's two day caucus retreat in Havelock North on Sunday, setting the stage for a bare-knuckles showdown in election year.
Speaking to media, Bridges said he did not trust New Zealand First and the public could not either.
"I don't believe we can work with NZ First and have a constructive trusting relationship," he said.
"When National was negotiating in good faith with NZ First after the last election, its leader was suing key National MPs and staff."
Bridges said it was clear a vote for NZ First was a vote for Labour and the Greens, describing the parties as "tied at the hip".
He said he was sick of the "charade" and wanted to provide New Zealanders with certainty and a clear shot.
"National won't work with New Zealand First after the election, full stop," he said.
"If New Zealand First has the balance of power, we still won't work with them."
The decision echoes one made by former National leader John Key in the lead up to the 2008 election, ultimately resulting in Winston Peters' ejection from Parliament.
Peters issued a statement in response, saying he was "unfazed" by the move and suggested Bridges was setting himself up to be rolled as leader.
"Let me say this - he's got a lot to learn about politics. Narrowing your options can be the worst strategic move you will ever make," Peters said.
"If Mr Bridges doesn't pick up the phone, someone else within his caucus will do it for him.
"As Douglas McArthur said: 'there'll come a time soon when he'll when want to see me much more than I want to see him'."
Door open to ACT
In contrast, Bridges left the door open to the ACT Party and again asked voters in the Epsom electorate to give their candidate vote to ACT leader David Seymour.
"National's had a constructive working relationship with ACT while in government," Bridges said.
"We developed the partnership schools model and worked together to reduce red tape. We would again be open to working with ACT."
In a statement, Seymour welcomed "National's encouragement" but said he would be campaigning again "as hard as ever".
"National recognises that it needs a strong ACT presence in the next Parliament in order to govern.
"The final polls of 2019 showed that ACT is likely to secure more MPs at the 2020 election and could make the difference between a Labour or a National government."