National's three leadership contenders have begun an intense lobbying campaign in a leadership battle that could go down to the wire over two weeks.

It looks set to become a two-horse race between Simon Bridges, who sees himself as the candidate of generational change, and Amy Adams, whose supporters describe as a woman of substance.

But veteran scrapper Judith Collins is also fighting fit and has vowed to hang on in the race even if looks as though she won't get enough support from colleagues.

"This is their chance," she said.

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"They can either do the status quo and more of the same. Or they can understand this is a real war, a real battle we are in and the National Party, if it wants to really be in there in 2020, we are going to have to do things a bit differently."

She did not intend to withdraw from the ballot, even if became clear she would not win. Nor would she leave Parliament if she lost.

"There's no way I'm going anywhere."

Bill English announced on Tuesday he would step down as leader on February 27 and retire from politics.

Paula Bennett and Nikki Kaye ruled themselves out from standing, although Bennett wants to remain deputy.

Adams, a rural MP from Selwyn, made a big visual impact by having four supporters behind her when announcing her candidacy, Nikki Kaye, Chris Bishop, Maggie Barry and Tim Macindoe.

She said she was born and raised in Auckland and grew up with a solo mother.

"We didn't have a lot of money but she instilled in me the understanding of hard work.

"I moved to Canterbury and fell in love and married my sheep farmer husband, Don. Happy Valentine's Day Don! This kinda wasn't how I was planning it."

Bishop said Adams had been a formidable minister in 10 different portfolios and described her as "a person of incredible substance."

Simon Bridges, aged 41, says he represents a mix of generational change and experience. Photo / Mark Mitchell
Simon Bridges, aged 41, says he represents a mix of generational change and experience. Photo / Mark Mitchell

Simon Bridges, the MP for Tauranga, said he would not be asking the MPs supporting him to publicly state their support - as many did when he challenged Paula Bennett for the deputy leadership in 2016.

"I'm conscious that we have got two weeks to run and I want to ensure we feel comfortable about having strong conversations in private about the future of our party."

He said he had strong support but it would be very competitive "and I'll be working very hard to give colleagues my pitch and hear from them what they want and need".

Bridges said he offered the right blend of generational change, but also experience.

"I'm 41. I have a young family. But I've held a raft of senior portfolios from energy, labour, transport and economic development, to communications and others besides. All of that gives me the experience, the acumen, and the drive to do this job very well."

Three other MPs are considering running: former Finance Minister Steven Joyce, former Health Minister Jonathan Coleman, and former Defence Minister Mark Mitchell.

Joyce said others had approached him saying he should stand.

It was about putting together the best possible team for 2020.

"I've never been particularly personally ambitious. That's not really my gig."

He said he had started in Parliament the same time as Bridges and Adams and there was not a massive amount of difference in terms of generational change.