Former Internal Affairs Minister Peter Dunne has urged National to "draw the line" on any potential alliance with New Zealand First ahead of the 2020 election.
Dunne, the former leader of United Future who retired from politics in 2017, said National's openness to forming an alliance with New Zealand First has "restored relevance" to the party.
"All that has done has been to confirm New Zealand First's status as a potential kingmaker once more for either the left or the right side of politics," Dunne said in a statement on Thursday.
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"In so doing, it has restored a relevance to New Zealand First that was increasingly lacking, as well as giving disgruntled National voters somewhere to park their vote, in the hope it might ultimately be of help in getting National back into government."
Dunne's comments follow National leader Simon Bridges suggesting to Magic Talk on Monday that he was open to forming an alliance with New Zealand First leader Winston Peters.
"If he is still in a position of being kingmaker at the next election, let's see what happens," Bridges said, referring to Peters, who ultimately decided whether Labour or National would form a government in 2017.
National got 58 seats in the 2017 election, 3 short of a majority. With New Zealand First's 9 seats, National would have had enough seats to govern - putting Peters in the "kingmaker" position.
But Peters chose to give his 9 seats to Labour, which had 45 seats, adding up to 54. That still wasn't enough, so Labour formed a Confidence and Supply Agreement with the Greens (7 seats) - forming the current Government.
Despite Bridges suggesting he was open to an alliance with New Zealand First, Peters slammed him the next day, telling Magic Talk he was confident National would elect a new leader before the next general election.
Peters said: "Simon Bridges isn't going to last much longer as the National Party leader - that's a fact. I've been around long enough to know."
Dunne, who has experience forming cross-party alliances, said: "Courting New Zealand First, the way Mr Bridges now seems to want to do, plays right into New Zealand First's hands, making them centre stage once again."
New Zealand introduced the Mixed-member proportional representation (MMP) system in 1994, and after that Dunne formed Confidence and Supply Agreements with both National and Labour as a support party - similar to Peters.
Peters formed a coalition with National in 1996, when he first held the position of Deputy Prime Minister under former leader Jim Bolger. But he was sacked from Cabinet in 1998 while Jenny Shipley was in power.
New Zealand First was also part of the Labour-led government formed in 2005, but by contrast it was a Confidence and Supply Agreement, like what the Greens have today.
Dunne said Peters' "driving motivation of the last 30 years has been to make National pay and continue to pay for the way it has treated him" during the 1990s.
He said National should rule out a potential coalition agreement with New Zealand First ahead of the next election, pointing to 2008 when former Prime Minister John Key "bluntly ruled out any deal" with Peters.
"It is a clear message to even the most obtuse National voter that a vote for New Zealand First is no more than a vote to re-elect the current Government," Dunne said.
"Such a declaration immediately would deprive New Zealand First of its ability to play both sides off against each other."