It's one thing for the likes of Blair McDougall to believe that the Corbynite left are to blame for increasing the chances of continued Tory rule by "forcing out" Luciana Berger and others. But whoever is ultimately at fault, there is no question about what the strategy should be for those who remain inside the party after a breakaway, assuming the priority really is to get the Tories out of office. To minimise electoral damage, you have to ruthlessly attempt to stamp out the new group, detract from the credibility of those who have joined it by accusing them of cowardice for not resigning their seats and fighting by-elections, attack them for betraying activists and breaching the trust of voters who elected them, etc, etc. We've heard that script time and again over the years whenever there have been splits of any size, or indeed even when there have been isolated defections. It works.
Bizarrely, though, the Deputy Leader of the Labour party is doing the complete opposite. Tom Watson's reaction video posted on Facebook after the breakaway must be one of the most extraordinary spectacles in the recent history of British politics. He barely even made a token effort to suggest that the splitters had done anything wrong (the best he could muster was that they had acted "prematurely"). He left viewers in no doubt that the splitters had the right prescription for the future, and that the current Labour leadership have the wrong prescription. As there is no prospect whatever of Jeremy Corbyn resigning as Labour leader any time soon or substantially changing course, the calculated effect of Watson's intervention is to aggravate the electoral damage of the split, rather than to mitigate it. With Labour having used the 2017 election to almost miraculously reverse the disunity and self-harm it had been indulging in for a year or longer, Watson has weirdly seized an opportunity to intentionally drag Labour back to its pre-election state. He's cheering on the splitters in the hope they will do roughly what the SDP did - ie. fail as a party, but remake Labour in its own image with the help of the fellow travellers who stayed behind. And as we all know, the SDP only 'succeeded' at the cost of an extra decade and a half of Tory rule.
Watson and others are self-evidently not prioritising the removal of the Tories from power, but instead are solely concerned with winning a Labour civil war, even at the expense of many more years of Tory government. The "moderates" will squeal with indignation at any accusation of "betrayal" from the left, but if your loyalty as Deputy Leader is to a faction rather than to the party you were elected to serve, you don't really have much defence against the charge.
The conclusion of the video saw Watson call for a more broadly-based Labour front bench - a not-terribly-subtle way of saying that Jeremy Corbyn has appointed the wrong people. In truth, there is nothing unprecedented about the lack of balance on the front bench - Tony Blair came from one extreme of the party just as Corbyn did, and he had only a sprinkling of token left-wingers in his team, all in junior positions (Chris Mullin, for example). But somehow the sidelining of people on ideological grounds is beyond criticism when it's the right that does it. Can you imagine what would have happened if John Prescott had posted a video in his time as Deputy Leader saying that Blair had appointed the wrong ministers? His position would have been instantly untenable. And in any normal party functioning normally, Tom Watson would just have made his own position untenable.