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At last count, Social Housing Minister Paula Bennett had 19 out of the required 30 votes from her caucus, based on public declarations.
Transport Minister Simon Bridges had 10 votes.
The ministers are competing to be deputy for Prime Minister-in-waiting Bill English, whose competitors Judith Collins and Jonathan Coleman dropped out of the race yesterday.
Ms Bennett's supporters are pointing to her Auckland base, the male and female balance and her good relationship with Mr English.
Mr Bridges' backers are painting him as a way to bring in a new political generation. Trade Minister Todd McClay has also pointed to the Auckland-raised Mr Bridges' provincial representation in the Tauranga seat.
Some caucus members were torn.
Auckland Central MP Nikki Kaye said her vote would go to Ms Bennett.
"I have been a bit torn because I rate them both very highly. But on balance I'm voting for Paula because I think she's a very strong leader for Auckland and will be a very complementary person for Bill.
"But I do think Simon has got a huge, huge future and it has been a hard decision."
Junior whip Jami-Lee Ross said he was backing Mr Bridges because he was a team worker and would back up English well.
"It's a good opportunity for the National Party to show it can deliver both stability and generational change for New Zealand."
Justice Minister Amy Adams said she was backing Ms Bennett because she was "hoping to see a male-female team taking us into the election".
"I think those two are a perfect complement for each other," she said.
Mt Roskill-based list MP Parmjeet Parmar said she supported Ms Bennett because she wanted an Auckland-based MP.
"[Ms Bennett] also originally came from a red seat and so she understands how hard I'm working in a red seat, putting my heart and soul into it."
List MP Brett Hudson said he was backing Ms Bennett because it was important to have a strong Auckland presence in the leadership team, and she complemented English.
"Paula Bennett has a phenomenal working relationship with Bill and will enable Bill to be the leader he wants to be, the Prime Minister he wants to be.
"That's important as a way to continue with the stability and reform that people are used to under this Government."
Mr Bridges has said he will champion the backbenchers if elected deputy, in response to concerns from junior MPs that too much power was concentrated among the most senior ministers in Cabinet. He has made a point of saying he is from outside the "Kitchen Cabinet" Bennett is a part of.
Ms Bennett has pointed to her close relationship with Mr English and her experience in the top tier of Cabinet. The two have worked together in the finance and social housing ministries, and on the Government's "social investment" approach for social services.
Mr English and his deputy will be sworn in on Monday after a caucus vote.
He has resisted endorsing a deputy, saying they were both "excellent contenders". Asked about his close relationship with Ms Bennett, he noted that he had also worked with Mr Bridges on solving Auckland's transport problems.
Prime Minister John Key has also refused to endorse a deputy.