Analysis - Simon Bridges isn't running out of luck. He never had luck. It's his caucus that is running out, running out of reasons to keep him on.

Photo: RNZ / Rebekah Parsons-King

The job description for Leader of the Opposition has only one KPI. Actually, it's a TPI - The Performance Indicator: perform well enough in the polls to maintain a credible prospect of leading the next government.

For a party like Labour, with coalition options, that might be mid to high 30s. For National, with only Act's one seat offering, the target has to be in the mid-40s.

Bridges inherited that position from Bill English via John Key and he's maintained it for more than a year but now it is slipping away. A score like this week's Colmar Brunton showing of 40 percent triggers the warning light. He is entering the danger zone.

Another lifeline for Bridges is also slipping away: The Capital Gains Tax. The government's strategically inept handling of the Tax Working Group proposals had offered up rich pickings for the National leader.

Leaving a raft of new taxes on the table for weeks invited National to dine out on examples of voters who could be stung by the tax. Surely that is about to end.

Labour's electoral survival instincts, bolstered by the heavy scepticism of New Zealand First, will likely mean farmers, businesses and KiwiSaver accounts will be exempt, as the CGT is whittled down to little more than a tax on residential property investors.

At that point Bridges, hunting for a wolf, has returned with a lamb and has little other prey to target.

If this were a good faith performance review in the normal run of employment law practice then it would be difficult to remove Bridges.

If you benchmarked him against others it would be hard to argue he has underperformed. The life expectancy of the Leader of the Opposition is often not very long.

Stroll the cemetery and read the political epitaphs of Jenny Shipley, Bill English (who was disinterred to die again), Phil Goff, David Shearer and Andrew Little.

That Bridges kept his party in the mid-40s for nearly half a term is itself worthy of engraving in stone.

Could another National MP do better?

But Christchurch has changed politics as well as New Zealand. Yes, it is out of Bridges' control - and that would be taken into consideration in our hypothetical good faith performance review. But not in politics.

The Christchurch terror attacks were not only one of the darkest days in New Zealand history, but they also propelled the prime minister to unprecedented international recognition.

David Lange, Jim Bolger, Helen Clark and John Key - they all fancied themselves as masters of international statecraft - got nothing like the adulation Jacinda Ardern has received.

Bridges can't compete with that. And he isn't competing.

Instead, he's left to flail in the shallow, sullied waters of domestic politics. It's one of the quiet secrets of politics that major issues are actually a lot easier to deal with than small, petty ones.

Many leaders will rise to the challenge of a crisis. In the Global Financial Crisis and the devastating Christchurch earthquakes, John Key's National government regularly polled over 50 percent.

Phil Goff, his opponent at the time, had to deal with his MP Chris Carter going rogue or Darren Hughes accused (although not charged) with sexual assault.

In the big scheme of things, the stakes are so small that you can never win.

Bridges has Jami-Lee Ross and the ripple effects of his rebellion. He has backroom staff, that no one outside central Wellington has heard of, complaining about his leadership.

Worst of all is National's review of culture, which Bridges called for in the wake of complaints from women in the party about Ross' behaviour.

Photo: RNZ / Claire Eastham-Farrelly

On Morning Report this week, Bridges wouldn't say who did the review and couldn't say whether any female MPs participated. He refused to reveal anything of its contents until Parliament's own review is released and then revealed he hadn't read it himself.

Again, it's small stuff in the grand scheme of things. But that is the point. While Ardern plays on the international stage dealing with matters of life, death and national security, Bridges is braying in the wilderness of party political irrelevancy.

I started by saying the reasons for keeping Bridges are slipping away. There are still reasons but they are more like questions - the doubts that will always accompany a change in leader.

Could another National MP do better? Are any of them likely to confine a Labour-led government to one term anyway?

No one knows the answers to those questions. If National is feeling cautious, that will help him. If they feel like taking a risk, that removes another reason for Bridges to stay.

I have a liking for cricket analogies and I'll end with one. Judith Collins is the next batter up. She has the pads on. She is a big hitter. She could crush, crash or burn. National needs to decide if it's going to keep going with the night watchman or send in The Crusher.