Despite the talk of a no-confidence letter, in the event that things got very bad for Labour, it is unlikely that Shearer would need a letter.
The more likely scenario is that party seniors and faction brokers Phil Goff and Annette King, would tell him: it's time, David.
He would resign and a new leadership contest would begin.
How bad would it have to get? There are two reputable polls set to be published in the next few weeks: TV One's and TV3's.
May's TV One and TV3's polls both had Labour on 33 per cent, and Fairfax's had it on 31.9 per cent. Including DigiPoll, three of the four polls have Labour heading south.
It is hard to imagine that the "man-ban" issue (allowing some electorates to have women-only selection contests) and subsequent coup talk has done anything but damage.
Shearer looked decisive for half a news cycle. The lasting image is not of a strong leader but one being kicked by one side of his party for not acting sooner and kicked by the other side of the party for acting at all.
The poor man is beginning to get the public's sympathy, not its support.
The question is whether that downward spin has begun from which there is no recovery.
If Labour were to drop into the 20s in all three polls, that would be understandable given recent events.
If it were to stay there for three months, it would be fatal to Shearer - Goff and King would come knocking. Then the battle would get interesting.
Would the party go with the candidate that could get them closest to Government but risk further disunity in the party, Cunliffe?
Or would it risk going with the lower profile deputy, someone less likely to get them into the Government, someone with less public appeal (nothing to do with him being gay) but more likely to unify the party?
This is the Robertson dilemma. He might be ready but is the public?
Labour's political management has been woeful, with a recent series of blunders including, but not limited to the Man-Ban, the SkyCity corporate box fiasco, getting too close to the Greens and the lingering discontent within the caucus that David Shearer has failed to temper. The voting public sees that, hence Labour's dreadful poll rankings.
How long will they let it go on? The next round of polls could be telling.