The MPs spent yesterday and last night in last-minute lobbying – including of each other – and will each get five minutes to make a pitch to caucus this morning.
Adams and Bridges are expected to be the frontrunners, although Steven Joyce claims he has a chance.
But neither have claimed to have the 29 votes needed in the bag – and sources within their camps are expecting the contest to be extremely close.
It will likely come down to the second choices of the supporters of Joyce and Collins.
The vote will be done in progressive rounds, with the bottom-polling candidate knocked out on each round and their supporters casting their votes for another.
The Adams' camp in particular had assiduously courted the "second" votes of supporters of Mitchell, Joyce and Collins.
However, with some MPs yet to make up their mind things could hinge on the last-ditch speeches the contenders will deliver to caucus this morning - a strong performance could shift the "soft" vote.
The deputy leadership contest is also in the air. While all have said they have made no firm commitments on positions, the leadership contenders are understood to have sounded each other out to assess the appetite for being deputy.
The most likely combination is Simon Bridges with Paula Bennett as deputy, or Amy Adams with Steven Joyce as deputy.
While it is likely the loser of Bridges and Adams will be offered the deputy role if the vote is close, both are likely to prefer the finance portfolio.
Collins has also said she would consider deputy and she as well as Jonathan Coleman also want finance.
Whatever the outcome, today's vote spells the end of the John Key and Bill English era – an era of unprecedented high popularity for National.
In the pitches today, Joyce is likely to warn National not to put all that at risk by going for a major change. Bridges and Adams too have been cautious about promising a wide-scale shakeup while Collins will be pushing for a more aggressive line in Opposition.
The new leader will be expected to keep polling high and discipline tight – which is not an easy task in Opposition.
None would commit to putting all of their rivals on the front bench but the front-bench posts and plum portfolios will be critical to try to prevent divisions in caucus – MPs such as Collins have made it clear they expect to be on the front bench.
Collins said if she was unsuccessful it would be up to whoever was leader whether she was put on the front bench. "But I would have thought it would be a bit of a waste of my skills.
"Bill English brought me back on to the front bench for a reason and it's been made pretty plain to me by others that we all think very highly of each other's abilities."
While some believed a negotiated outcome would have been tidier, MPs spoken to said they were glad it was going to the full vote, for it meant the leader would be caucus' choice.
Joyce said losing English would be a major change and the focus for the new leader would be ensuring the right combination was in place.
"Irrespective of who wins, one of the challenges for the new leader is to draw the team together and make sure the change is a positive change. The one or two at the top, they carry a lot of the workload."
He would not rule out going for deputy if he was unsuccessful in the leadership.
"Should that not come to pass - and I'm still confident it could - I'm prepared to serve in terms of what the caucus wants."
All candidates have ruled out promising positions for votes in the lead-up to the election – but said they had talked to others about what shape a leadership team might take.
While it is likely they will offer some of their rivals front-bench positions, none would commit to it in advance.
Adams said they were talented and she would use all in the best way possible
Adams said they had been talking to each other but had not made any "deals" or promises in return for votes.
"It's quite natural we are having conversations, as we are with the core people in other peoples' teams because we are all after their first and second preferences and we are talking about how it might shape up. That's a pretty normal and healthy part of what's happening. But it's certainly not at the point of lockdown arrangements in place."
Mark Mitchell, the rookie of the pack, is unlikely to make the leadership team but the contest will have done him no harm. It was a good profile-raising exercise for him and leaves him well-placed for another tilt should today's choice be unsuccessful.
How it works
• National MPs will meet at 10.30 this morning to elect a new leader and deputy leader.
• Four contenders are Amy Adams, Simon Bridges, Judith Collins, Steven Joyce.
• New leader needs support from at least 29 of the 56 MPs.
• Lowest candidate is knocked out in each round of voting.
• MPs vote again until one candidate has at least 29 votes.
• Numbers are not revealed to caucus.
