Say what you like about Labour's Annette King, she knows how to stage a backflip with grace.

King's decision to quit as deputy leader and resign from politics at the election came just three days after King insisted she would not be going anywhere, thank you very much.

It was quite the turnaround from King's initial reaction to the suggestion Jacinda Ardern might now be a better deputy for leader Andrew Little.

She announced her decision to step down without a trace of bitterness. She threw her support behind Ardern - saying that upon reflection, she was doing what was in the best interests of the party.

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It also came three days after Little said he was not planning any changes - although even then there was a silent "yet" hanging off the end of the sentence.

There will inevitably be speculation about whether Little strong-armed King into it.

He may not have tried to talk King out of it, but it is doubtful he pushed her.

King and Ardern offered very different points of value as a deputy leader, but on balance there was little between them from his point of view.

It is more likely that after her initial anger cooled and the calmer heads of colleagues prevailed, King came to the realisation herself that Ardern was a good fit in an election year.

The key words here are "in an election year".

Some believe Ardern should have been given the role earlier. That is codswallop.

Little appointed King for good reasons. Little was elected by the unions and members rather than caucus. There was not antagonism toward him but he did not have a strong, loyal core support base in caucus as a relatively new MP.

King gave him hers - holding the line while he got on with making the mistakes every leader has to make to learn, making the decisions every leader has to make and enjoying the occasional tribulation.

It was as much due to King as Little that the outbreak of unity in Labour finally happened after six years of problems.

Ardern did not have the influence or heft of King over the caucus to achieve that.

What Ardern does have is what Little needs now: she is an electable asset.

The deputy role can be defined by a leader to meet what is needed at any given time. Ardern will be a campaigning deputy.

Others will step in to do some of the backroom work King did. The role of caucus disciplinarian will likely fall to Little and whip Kris Faafoi. Finance spokesman Grant Robertson and Education Spokesman Chris Hipkins are also Wellington-based and can dip in where needed.

King's decision was also likely prompted by King's reflection that for years she had been Ardern's mentor and maybe it was time to allow Grasshopper to step up.

Whatever the merits of her decision, Labour has lost one of its giants.