John Boscawen says he put his hand up for the ACT leadership once Rodney Hide made it clear he wouldn't stand.
Boscawen had tried to persuade Hide to return to the job and had visited him in Wellington and Christchurch.
Hide was rolled by Don Brash in 2011 and the job is again vacant because the party's sole MP, John Banks, will face trial this year over campaign donations.
In a surprise announcement this evening, Boscawen said he wanted to contest the Epsom seat and lead the party.
"There was a view among quite a few of our members that they would like to see Rodney back," he said. "Had Rodney [put himself forward] I wouldn't be."
Until now journalist Jamie Whyte was the only candidate for the top job and former minister Boscawen said he wanted to offer the party's board a "real choice".
"I think I'm the best choice ... I actually believe that the leader of the party should be the [Epsom] candidate ... we must put our best candidate up to the people of Epsom," he said.
Whyte, a former Cambridge lecturer, has also put himself forward for the Epsom nomination but "has expressed a preference just to do the leadership role", Boscawen said.
At a December press conference, Boscawen said he had not regretted quitting Parliament in 2011.
He revisited this over Christmas.
"I think it [being and MP] is the highest former of service, the highest form of charity," he said.
Boscawen is a major funder of the party but he insisted this wouldn't sway the board. He will step aside as president until the board makes a decision early next month.
He insisted he wasn't approached by coalition partners National to stand. However, Boscawen admitted it would be easier for Prime Minister John Key to negotiate with someone he knew.
Boscawen was a key broker in previous coalition deals.
Also in the running for Epsom is list member David Seymour. Former president Chris Simmons has also expressed an interest in the Epsom nomination but not made a formal declaration.
Whyte said Boscawen made him aware of the decision today.
He insisted there had been "no falling out" and he was supportive.
"I was planning to ask him if he would do it, and then he told me that he was," Whyte said.
"So, that was a surprise. It may just be that the party was in a bit of a lull and the party has taken off again and he's put himself forward. He's very engaged and maybe he is enthused."
Boscawen has been an ACT member for 17 years and entered Parliament on the list in 2008. He contested the Mt Albert by-election in 2009, during which he memorably wore a lamington, thrown from the crowd at a hustings meeting.
- © Fairfax NZ News
Should Judith Collins resign?