David Lange could certainly do it. He could turn it on and off like a tap. Helen Clark could do it when she had to do it. And then she would lay it on thick. Mike Moore could do it but then he would go and overdo it.
"It" is the capacity shown by Labour leaders to draw strength from the party's founding principles of fairness and equity and its proud track record in delivering policies of such complexion as a means of uniting and galvanising the party.
In an age of sound-bites and software, the leader's address to his or her party's annual conference might seem old hat.
But it remains the ultimate test. A poor speech by a struggling leader can sound like a death-rattle. A good speech can silence the critics - at least for a month or two.
The best speeches raise goosebumps, tingle spines and place lumps in throats.
They are emotive affairs, their content being directed with the unflinching accuracy of heat-seeking missiles at the heart as much as the mind.
The audience is a willing partner in this manipulation. But getting delegates to fever pitch requires the leader to dig deep and display real passion - and in quantity.
The wording can be manicured by speechwriters, but the spirit driving such a speech's content cannot be manufactured out of nothing. It cannot be faked. It requires an unquestioning belief in the absolute correctness of the cause. There can be no room for self-doubt.
The capacity to rouse the party faithful in such fashion is the one thing which separates the great Labour leaders from the rest.
Sir Geoffrey Palmer was too much in the shadow of Lange. Phil Goff thought he could do it, but could not shrug off his incriminating past as a one-time acolyte of Sir Roger Douglas and his guilt by association in Lange's downfall.
So can David Shearer succeed where Goff failed? The vast majority of the 600-plus delegates inside Auckland's Ellerslie Convention Centre will be collectively willing Shearer to do so when he mounts the podium tomorrow to deliver his speech to this year's Labour conference, his first as leader.
Labour loves to wallow in its history. Housing policy has been very much part of that history. So it is not surprising that it will be a major theme of the speech.
Even so, the betting is heavily weighted as to which side of the fence Shearer will fall in terms of the ultimate test.
Shearer is a capable set-piece speaker. The stumbles in stand-up press conferences that send unintended messages of doubt and hesitation to radio and television audiences are largely absent from his delivery.
But the expectations being layered onto tomorrow's speech are daunting even if he shifts up a couple of gears - something he must do anyway.
The pressure has been exacerbated by the extent and timing of this week's calls for Shearer to step down. He needs to ease some of the pressure before the speech by stamping his authority on the conference rather than passively allowing the media to play a mischievous and destabilising Game of Two Davids all weekend.
Shearer needs to dictate the media coverage today as well as tomorrow and ensure that neither David Cunliffe nor those gunning on his behalf get a look in.
Shearer's leadership may not be under immediate threat. But those who want him out may turn today's rewriting of the party's constitution - particularly proposed new rules covering the election of the party leader - into something of a symbolic battle where delegates' statements and amendments to those rules carry a double meaning.
The organisational review is one of the largely unseen steps that have been taken under Shearer's watch to modernise and revitalise the party. That exercise has been backed up by a push to regain lost territory in the provinces.
One big plus for Shearer is that the conference is happening at a time when Labour has some grounds for thinking the debate on economic management is shifting in its favour.
Labour is already sloganeering, saying National is driving the country down the wrong track and Labour's more interventionist approach is now the right track to follow.
That is language everyone can understand. A priority for Shearer is explaining how the bits and pieces of Labour's policy jigsaw - be it a capital gains tax, raising the retirement age to 67 or requiring the Reserve Bank to take heed of the high exchange rate - fit together and where they will take the country.
But doing battle with Bill English is for the long haul. The more immediate concern is that Shearer drastically lift his own game.
He is someone who prefers to be seen as part of a team - someone who shares the credit rather than grabbing it all for himself. He believes achievements speak louder than words.
But in his job, meaningful achievements are few and far between. Words - of which there is no shortage - have to be spoken at full volume to be heard.
There can be no more fudging. Shearer must reverse Labour's tendency to be over-cautious when it comes to taking a position or drawing a line in the sand.
Such blurring of the ideological boundaries between the two major parties serves only to make New Zealand politics more presidential.
Labour initially sought to protect its inexperienced leader by positioning him above the political fray as some kind of non-politician politician. That was never a goer. It left Shearer playing catch-up in terms of exposure.
In that regard, Mr Nice has reached his limits. The voters now need to see flashes of Mr Tough. Shearer's language has to become more cutting, more emphatic and more definite - and thus more quotable and more memorable.
He must get more bite from what is now a pretty wimpish bark. If that means grabbing issues off colleagues and running with them hell-for-leather - Clark's profile-raising technique in opposition - so be it.
The selfless Shearer must become the selfish Shearer - not just for his sake but for the party's sake, because the leader embodies the party.
This weekend, Shearer needs to show who is the boss within Labour. We should know by tomorrow afternoon to some large degree whether he has succeeded. Or whether February's vote of confidence in the leadership - decreed under caucus rules to take place in the middle year of the parliamentary term - is likely to shape up as not being quite the formality it usually is for the incumbent.