Angela Rayner has just dealt a devastating blow to Keir Starmer

Question of the day: what is the job of a deputy party leader? Previous incumbents of the post in the Labour Party never doubted the answer: it was to support the leader publicly and to advise him privately. That was how John Prescott, probably the party’s most successful deputy to the party’s most successful leader, saw it.

Prescott was a man of the Left but he had a keen understanding of pragmatism and the role it plays in shaping a party’s platform and helping it to victory. Deputies often have to bite their tongue and smile for the cameras. Self-indulgence, that most tempting of political traits, should always take second place to the needs of the party.

Angela Rayner seems to have a different approach, and it’s one that threatens to dangerously undermine her own leader. And for “leader”, read “party”. For during an election campaign – especially during an election campaign – the leader and the party might as well be the same thing. What’s good for the leader is good for the party. A disaster that afflicts the leader is a disaster inflicted on the entire party.

Rayner, perhaps feeling emboldened thanks to being cleared over her alleged avoidance of capital gains tax, has weighed in on the Diane Abbott controversy, which over the past 48 hours has seen Labour’s professional, calm, measured and uninspiring campaign descended into chaos.

What Starmer needed was for some high-profile Labour people to weigh in on his side, supporting him in his desire to clean out the Augean stables of the Parliamentary Labour Party once and for all, to congratulate him for going further than any of his predecessors in cleansing the party of extremist views. Because without such interventions, Starmer risked looking weak, isolated and – worst of all for any aspiring prime minister – not in charge of his party.

Alas, Rayner, on whom her boss should be able to depend for support, today refused to offer it. Instead she took the position that she knew was the direct opposite of Starmer’s: “I don’t see any reason why Diane Abbott can’t stand as a Labour MP going forward… I am saying that as the deputy leader of the Labour Party.”

Without wishing to sound too apocalyptic on the matter, this is devastating for Starmer. The very fact that Rayner added that she was stating her opinion as Starmer’s deputy is a direct challenge to his authority.

There was no need to do so. She could have equivocated. She could have passed the buck elsewhere. She could have claimed that her taking a side, one way or the other, would only prejudice the case.

More to the point, she knew that any of the above options would have helped and strengthened Starmer. And Angela Rayner chose not to do that. She decided to play to the gallery, to activists rather than voters, to the media rather than her boss.

In doing so, she has risked damaging irreparably the relationship between her and Starmer. If he becomes prime minister in July that will be bad news for her own career prospects. She cannot be sacked as Starmer’s deputy, but it is up to him what job in government she gets, or if she gets one at all. And a victorious party leader will be given great, almost unlimited, leeway to make controversial personnel decisions, especially in the early days of his administration.

Presumably Rayner knows this, but went ahead with her message of solidarity to Diane Abbott anyway, despite Abbott’s record of opposing the Labour whip (even when she had it) and despite her clumsy and ignorant claims about Jews – the sin which triggered her suspension from the whip in the first place. In Rayner’s judgment, parliament and the Labour Party would be better with such a person in its ranks.

Starmer is one of the least politically experienced leaders of the opposition in history, but he knows enough to recognise treachery when he sees it. His reputation for ruthless dispatch of his political enemies has led him to this current pass; Angela Rayner must hope that, for her own career’s sake, that ruthlessness will be blunted before he gets to Downing Street.