National expected Peter Dunne to "stay and fight" for the Ōhāriu electorate but had factored there was every chance he wouldn't win, says campaign manager Steven Joyce.
Mr Dunne has decided to withdraw from the election, rather than face the prospect of being beaten after 33 years as the local MP. His United Future party has given National support on confidence and supply and in some cases the vote needed to pass significant legislation over the last last three terms.
"We expected him to stay and fight - there's a bunch of letters in people's letterboxes all over the Ōhāriu electorate as we speak," Mr Joyce told Morning Report.
But National was not on the back foot and Ōhāriu was now a "two-way drag race" with Labour. "There's now some pretty clear lines and competing visions for New Zealand."
Mr Joyce said he'd concluded a couple of weeks ago that Mr Dunne would struggle to retain the seat.
"Peter was obviously worried about his own ability to win and I think that's fair - you've seen the polling numbers. We'd factored in the fact that there was every chance he wouldn't win."
He would not "prejudge" whether National was now likely to need New Zealand First and its leader, Winston Peters, to form a government.
"We don't know whether the Greens will be there, we don't know what the major parties' votes will be, we don't know what the New Zealand First vote will be - so I'd just say give it a little bit of time."
United Future deputy leader Judy Turner said she had no prior warning Mr Dunne was stepping down.
"I didn't find out about Peter's resignation until yesterday afternoon," she told Morning Report.
"I think he was probably focusing on his own future, to be quite honest with you ... and he has every right to."
She said Mr Dunne had spoken to the party president and there had been signs he'd "taken his foot off the pedal" in terms of campaigning.
United Future candidates would continue stand in the election and members would wait until after the election to decide whether to continue as a political party, she said.
Mr Dunne said he had been mulling the decision to step down for some time, but it was the change in the political mood in his Ōhāriu electorate over the last month that made him realise his "battle was going be too uphill".
"Frankly, it came down to 'did I want to'," he said. "That was really when I woke up to the fact that, when the crunch came, my heart wasn't all that much in it.
"I thought it was much easier to make the move now and allow the electorate to make the choice, that's a clean one."
No-one would want to end their career with a loss, he said.
"After 30-odd years you want to retain your dignity, you want to have your head held high and you want to leave on a positive note. I came to the conclusion that was far more appropriate than fighting an election campaign and then having to make that awful concession speech on election night if it didn't turn out."
His decision was not a betrayal of National, he said. He spoke to leader Bill English yesterday morning, a little before he made his announcement.
While there was a lot of volatility at the moment, things would settle down closer to the election, and there would be a clear choice, he said. "I think it will be the experience, judgement and the calmness of Bill English versus the alternative and I think on election day you'll see that he comes out on top."