It was astonishing. It implied treachery in the extreme. What the outburst really revealed was someone looking for scapegoats for his own self-inflicted woes.
David Cunliffe actually stopped one step short of uttering the word "scab" during his appearance on Campbell Live on Wednesday evening. But he noted that in the Labour movement "there are words we use for strike breakers". He meant one word. And you did not have to be Einstein to work out what that word was.
Likewise those MPs in Cunliffe's sights who must be furious at being labelled in such derogatory fashion.
In fact, Cunliffe spent much of the week trying to play the victim following the embarrassing revelation that he had helped Donghua Liu with his application for New Zealand residency, having just 24 hours earlier denied any such advocacy on behalf of the controversial Chinese businessman.
Cunliffe countered that National had set him up, having known for weeks about the letter he had written back in 2003 to immigration authorities on Liu's behalf.
It is true National was well aware of the letter, but only because it had conducted a document trawl to find out more about Liu after he proved to be of major nuisance.
John Key says he did nothing with the letter as it did not seem particularly germane to anything at the time.
That is difficult to accept. The letter would have looked like a gift from God - especially as its contents cut right across Cunliffe's "crony capitalism" campaign.
If Cunliffe was stitched up, he compounded things with his denials of any contact with Liu.
It begs the question of why Cunliffe has fronted so much of Labour's over-exaggerated efforts to paint National as a Government of sleaze, decadence and corruption rather than standing above the fray.
The answer is that he is caught in the dilemma that faces most Leaders of the Opposition: do you keep out of the day-to-day political squabbling only to end up being accused of being invisible or do you lead from the front and end up with not just getting your hands dirty, but your reputation dragged through the mud?
Inevitably, the upshot of the Liu letter and Cunliffe's musings on "strike breakers" only further highlighted the lingering factional fight in the Labour caucus between those who went out on a limb in backing the somewhat accident-prone but potentially game-changing Cunliffe and those who would prefer the less exciting, but safer Grant Robertson in the leader's job.
So far, there have been plenty of accidents, but no game-changer. And it is hard to see where one is going to come from in the three months left until the election.
The media, meanwhile, has dug out Labour's recently revised constitution and its special provision which stipulates that a leadership election within three months of a general election is restricted to the caucus - as in the past - rather than also taking in wider party membership.
This rule was included to deal with the sudden death or incapacity of a leader close to an election when there is insufficient time to conduct vote of all party members. It posed the obvious question: would the caucus use this as an opportunity to dump Cunliffe?
If that and Cunliffe's blunder was not enough to set the leadership drums beating, a Fairfax poll had Labour's support slumping to a dreadful 23 per cent and sending a cold shiver down the spines of electorate-less list-only MPs dependent on a healthy party vote.
By Thursday morning, things looked like they might spin out of control. Cunliffe, who usually spends Thursdays out in the regions, flew from Auckland to Wellington for what was swiftly (and accurately) dubbed as a "crisis meeting" of senior MPs and staff in Labour's wing at Parliament.
No matter what faction of the caucus they were from, everyone present would have been able to agree on one thing: disunity is death.Voters hate it. And punish it.
Labour polling in the low 20s percentage wise was in no one's interest, pro-Cunliffe or anti-Cunliffe. If Labour replicated National's disastrous showing in 2002 when that party got just over 20 per cent of the vote, Labour would be potentially looking at another two terms in Opposition.
National's vote collapsed in 2002 in part because the polls showed that it had no chance of winning. Centre-right voters consequently voted tactically for New Zealand First and Peter Dunne's United Future in the hope those parties would be part of a governing arrangement and thereby constrain Labour from adopting and implementing policies of the left.
The good news for Labour is that the centrist options are no longer so much of a threat, United Future is no longer registering in some polls, while New Zealand First has struggled to make much of an impact in the current term.
The worry for Labour is that its vote may just be collapsing through people simply losing patience with the party.
The priority at Thursday's meeting was to paper over the cracks and restore a semblance of unity by killing off the leadership speculation, however far fetched it might have been. Robertson duly did so by stating pretty categorically after the meeting that he would not be mounting a challenge before September's election.
It would have been an easy thing to say. Cunliffe has steered Good Ship Labour on to the rocks. There is no point in Robertson also being part of the election-day wreckage.
