The only serious contenders for the sudden vacancy in the National Party leadership are Judith Collins and Simon Bridges.
They are the ones with the lowest risk 10 weeks out from the election.
The two other possibles, deputy leader Nikki Kaye and Mark Mitchell, would be not only high-risk, they would never get the numbers. They should rule themselves out now.
Amy Adams and Paula Bennett are automatically ruled out. You cannot announce your retirement from politics then credibly return as leader.
Gerry Brownlee would be leader only if a warring caucus had torn itself apart and he was the last man standing.
There is one simple way of choosing between Bridges and Collins and that is to work out which of them Labour would most fear as National leader.
The answer is not Simon Bridges, despite him having done much better in his 27 months as Leader of the Opposition than most rivals gave him credit for.
Bridges and his supporters have already had a moral victory, albeit a Pyrrhic one, with the resignation this morning of his ouster, Todd Muller.
There has been some apparent nostalgia for Bridges on social media in recent weeks, as Muller has stumbled.
But for the most part it has been faux nostalgia, amplified up by the Left rather than genuine nostalgia.
The answer to today's problem is rarely yesterday's leader. Bridges might win another contest but it is by no means certain that those who supported him in May will necessarily back him now.
Judith Collins is untested in leadership but through her own crises she has shown survival instincts as strong as Bridges, and was a highly competent minister.
She has a huge public profile and as her recent book, Pull No Punches showed, she is a complex mix of vulnerability and steeliness.
She is the only one likely to create anything like enough momentum for National to bulldoze its way out of this swamp.
She does not have what would normally be called a support base in the caucus but Muller's liberal supporters and MPs in precarious positions may weigh in behind her as the better gamble.
Because Collins does not have a traditional support based in the caucus, no more than a handful, she comes with little baggage. She doesn't have loyalties to repay. She doesn't owe anyone.
She would be better placed to make it a Collins-Bridges team, perhaps with finance spokesman Paul Goldsmith as deputy and Bridges at spot three or four, if Bridges didn't want to be deputy himself.
One of the mistakes Bridges made when he took over was in keeping Muller so low in the caucus rankings, and one of Muller's first mistakes was in keeping Bennett and Bridges so far removed from the front bench.
There have been many lessons in National's first term back in Opposition. Whether they have been learned or not will be evident in the next few days.