Peter Dunne's sudden departure from politics in the face of certain defeat in his electorate of 33 years should make no difference to the final election result in theory.

That's because the Party vote determines the make-up of the Parliament, not electorate seats like Dunne's Ohariu seat.

But the optics are terrible for National and Bill English, who have put a lot more effort into endorsing Dunne at this year's election than John Key did previously.

Dunne may not be a rat but it looks like he is abandoning the centre-right ship.

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Dunne and his United Future party of one have been valuable to National at the margins.

And the margins are where elections are won and lost under MMP.

Dunne is the overhang MP in the current Parliament.

That means the current Parliament is 121 MPs instead of 120. And that has meant a slightly larger cushion for the National-led Government.

However, the tide was running so badly for Dunne in Ohariu that he almost certainly was going to lose the seat and not provide any cushion.

National's candidate and list MP Brett Hudson may now campaign for the electorate vote, but it is really only the Party vote that National still wants.

Dunne has been part of every Government under MMP except the first two, the National New Zealand First one and the Labour Alliance one.

But from 2002, when his party gave Helen Clark's Labour Government confidence and supply, until and including three National-led Governments, he has been a fixture on the political landscape.

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