The Labour Party is deluding itself if it thinks it has a choice when it comes to electing David Shearer's replacement as leader.

There is only one option. He may wear his super-sized ambition on his sleeve. He may have an over-inflated opinion of his own worth. He may be extremely unpopular in some quarters of the Labour caucus. He may even self-destruct as leader.

But David Cunliffe's time has finally arrived. There are plenty of reasons. But one stands out from the rest. His being Labour's leader worries National far more than Grant Robertson, the only other viable contender, getting the job.

Cunliffe is the closest thing Labour has to an X factor.

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That is not to belittle Robertson, whose capacity to do the job is not in question. But Robertson is a left-leaning party technocrat from the Wellington metropolis who has yet to prove he is more than a Beltway politician and - regardless of him being gay - can make an impact in provincial New Zealand.

In contrast, Cunliffe effortlessly articulates a passion for the Labour cause which might well make people sit up and finally take notice. He can achieve cut-through to a watching audience on television. He is a very effective debater in Parliament who would take the fight to John Key.

Cunliffe is capable of pitching Labour's message leftwards, at the same time reaching rightwards to centrist voters who are currently happy enough with Key's pragmatic style of governing.

Cunliffe's first priority would be to rebuild Labour's brand and stop the Greens cannibalising its core vote.

Russel Norman has been allowed to become the de-facto leader of the opposition. Cunliffe would seek to reassert Labour's authority and make it clear who calls the shots on the centre-left. If that required giving the Greens a few verbal slaps, so be it.

The obvious question is whether his own party will buy the Cunliffe package, warts and all. The best option would be for Cunliffe and Robertson to combine as a ticket with Cunliffe being the candidate for the party's top job and Robertson being his deputy.

Such a ticket would be unbeatable. Other contenders would opt out of the contest, thereby avoiding an acrimonious three to four weeks - the time needed to elect the leader under the party's new rules which give the grassroots membership and affiliated trade unions a major say, rather than the decision as now being solely the caucus's prerogative.

If Cunliffe then manages to defeat Key at next year's election, all well and good. If he does not, then Robertson should get his chance. His worry is that if he waits for the leadership to fall into his lap, the likes of Andrew Little may sneak up and snatch it from him.

You can be guaranteed of one thing: Labour MPs' phones will be running white-hot this weekend.