Labour cannot afford that. But that cannot be allowed to happen. And for one very simple reason. The brute political reality is that support for David Cunliffe has snowballed to such an extent - and public expectation along with it - that the party's top job can no longer be really denied him.
MPs who have waited until the last minute to vote will have recognised that. That has put the onus on them to switch their allegiance to Cunliffe for the party's sake, if nothing else.
For all of Robertson's many talents which are complemented by a shrewd political brain, he lacks the one vital attribute which Cunliffe possesses in abundance - political chutzpah. In other words, supreme self-confidence and an audaciousness to take the kind of risks Labour will have to take to win next year's election.
It isn't just a question of taking the fight to John Key in the relatively short period before then. That is something of which Robertson is well capable.
Neither is it a question of who is best at unifying the party - an absolute priority and something which Robertson has repeatedly stressed he is best placed to achieve.
That no longer necessarily holds. A Robertson victory looks increasingly counter-productive to that objective.
What counts more than anything else is the new leader's ability to reconnect Labour with the hundreds of thousands who did not vote in 2011 and the tens of thousands who voted National when they had previously inclined Labour's way.
Robertson's ability to convince members he can do that has been overshadowed by the "gay factor". That has dogged his campaign. The novelty of a gay prime minister meant it was always going to do so. It won't the next time Robertson is vying for the leader's job. That will be old news.
Or at least it will if TV3 stops exhibiting a Victorian-era style prurient obsession with Robertson's sexuality which verges on the homophobic. The channel has seemed to think it has been given some God-given right to manipulate the result of the contest. Its self-appointed role as kingmaker is an insult to the intelligence of the Labour activists who are well capable of making up their minds without TV3's advice.
But that is another story. The one to be told tomorrow will focus on the scale of the successful candidate's victory. Along with the result, the party organisation will produce a breakdown of how each candidate performed in each of the three parts of the electoral college - the caucus, the trade union affiliates and the rank-and-file membership.
This may have been a not-so-subtle means of placing more pressure on MPs to fall into line with the wider party's wishes.
The biggest beneficiary of the release of this data is likely to be the National Party which will be able to highlight Labour's splits and divisions in exact terms from here to the other side of the election.
That data will be a millstone around Robertson's neck if it reveals he secured a narrow victory because his colleagues could not stomach Cunliffe.
But his even bigger handicap is that he remains an unknown quantity to most voters and very much the party apparatchik to those who are aware of him.
He rates far behind Cunliffe when the pollsters ask people who they think has the best chance of ousting John Key.
Robertson demonstrated during the round of meetings that the leadership contenders had with party members that he can articulate a vision - and with some real passion. But this has failed to come across publicly.
The brutal fact is that David Shearer soaked up too much time in trying to establish himself that Labour can no longer afford to devote any more of that precious commodity to raising Robertson's profile.
In contrast, Cunliffe is already the complete package. Driven by an over-powered ego, it may not be one that has endeared him to his colleagues in Parliament.
There is always a "cringe factor" with Cunliffe. But his crucial attribute - and one which has made him the darling of the Labour left - is a rare ability to define, articulate and then communicate Labour's basic values and message in clear and concise terms while also giving that message the power of moral suasion to shift public opinion Labour's way and leave Key and National isolated.
In short, Cunliffe is the potential circuit-breaker that Phil Goff and Shearer were not and which Robertson still falls short of becoming.
Cunliffe knows full well that rebuilding his colleagues' confidence in him is a priority. He will not be surrounding himself with fawning caucus supporters and handing them the plum jobs.
The dumping of Jennie Michie as his campaign manager for some harmless remarks about Robertson being gay was a signal to the caucus that he intends to act in good faith.
At the end of the day, however, the best way of building unity inside the caucus is for the leader to be successful outside.
Cunliffe is an example of what is described in political psychology circles as the "zeitgeist theory of leadership"- someone who is elevated to the role because the party's situation demands his qualities and skills.
The classic case - and Cunliffe will relish the comparison - is Winston Churchill in wartime Britain.
However, once peace was achieved, Churchill was dumped by the British people who perceived different qualities were needed to rebuild their nation.
The more relevant precedent in Cunliffe's case is Mike Moore. He saved Labour from destitution and brought the party to near victory. His reward was to find himself on the wrong end of a leadership challenge by Helen Clark, seen as the safer bet for Labour in the long term.
In Cunliffe's case, simply replace Clark's name with Robertson's.
