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Shortly after Harriet Harman announced her resignation as Labour’s deputy leader in 2015, John Healey, the MP for Wentworth and Dearne in South Yorkshire, took an old friend in the Parliamentary

Labour Party (PLP) out for a drink and asked for his support. In the race to become deputy leader, as in the leadership contest, members would have the final say – but first an MP needed to be nominated. To stand, a candidate required the signatures of 15 per cent of their parliamentary colleagues.

Healey is one of the most popular people in the PLP, but this conversation ended on a familiar note. The nomination would not be forthcoming. Nothing personal, you understand: it was just that another candidate, Tom Watson, had already asked.

When? “Three years ago,” came the embarrassed response.

Just because there isn’t a vacancy, it doesn’t mean there isn’t a contest. That was Watson’s great insight and it partly explains why he ended up as deputy leader while Healey didn’t even make the ballot. So it is with the top job. The Labour leadership is Jeremy Corbyn’s until he decides otherwise, but the race to replace him has started.

Even the leader’s allies acknowledge this and have started planning the succession to safeguard his legacy. As he is still marginalised within the parliamentary party, securing a keeper of the Corbyn flame is considered essential.

There are problems, however. One of them is the sudden rise of Clive Lewis, who has been identified with the Corbyn project since his election as the MP for Norwich South in 2015. Lewis resigned as shadow business secretary so that he did not have to vote for Article 50. His decision was praised by the journalist Owen Jones, a favourite with the grass roots, on Twitter: “Clive Lewis is genuinely one of the most principled people I’ve ever met, he resigned on a point of principle and that is commendable.”

Privately, the leader’s inner circle was irritated by this. Just as David Cameron’s team both acknowledged the star power of Boris Johnson and bemoaned the troubles that his Telegraph column created, so the high profile of the Guardian’s own blond firebrand is a mixed blessing. One senior Corbynite laments that Lewis has “had his head turned” by Jones’s advocacy. Others around the leader see Lewis as to blame for not doing more to dampen down talk of his leadership hopes.

Whoever is responsible, every move that Lewis now makes is seen through the prism of the succession. His resignation had far more to do with retaining his marginal seat in Norwich, which opted to remain in the European Union by a heavy margin, than it did with playing to pro-European sentiment in the party’s grass roots. Yet, because speculation about his future has blossomed unchecked for months, it was inevitably seen as part of a leadership bid.

In response, bookmakers slashed Lewis’s odds, making him the favourite to succeed Corbyn. In reality, his chances of getting the top job have lessened as a result of his decision to quit. The Labour leadership is irritated that he did not resign in the swift and subdued manner of the other shadow cabinet members, whose consciences – or, if you are cynical, constituents – made voting for Article 50 impossible. Instead, Lewis publicly pondered his fate, depriving the party’s amendments to the government’s Brexit bill of the oxygen they needed to thrive. He is therefore unlikely to be anointed Corbyn’s heir or to inherit his institutional support among trade unions and Momentum.

As far as Corbyn’s closest allies are concerned, the great hope is Rebecca Long-Bailey, another member of the 2015 intake and Lewis’s replacement in the shadow cabinet as shadow business secretary. Senior figures in the leader’s office, the shadow cabinet, the trade union movement and Momentum have long been extolling Long-Bailey’s virtues. After she appeared on Question Time on 2 February, John McDonnell hailed her as the “next generation of our socialist leadership team”. However, it is not clear if she is willing or ready to step up so soon.

So, who might back Clive Lewis? The 51 Labour MPs he joined in defying the whip over Article 50 are no basis for a leadership challenge to Corbyn from the left. As one of the dissidents observed, the Europe rebels are “a scale model” of the Labour Party, with every faction represented among the 52 rebels. What unites them is either that they sit for a constituency that backed a Remain vote, or that they will be well past retirement age at the next election.

In any case, much of the debate about whether or not Corbyn was right to instruct his MPs to back Brexit ignores how the overwhelming majority of the PLP believes that the Leave vote has to be honoured, not defied. Opposing Article 50 might win over party members, but it terrifies Labour MPs, some of whom fear going the same way as their former Scottish colleagues, who lost their seats after ending up on the wrong side of the referendum divide.

That is why the shadow Brexit secretary, Keir Starmer, and the shadow foreign secretary, Emily Thornberry – both of whom are believed to have leadership ambitions – were among the most vocally supportive of voting to trigger Article 50. It is Thornberry who provides the best insight into the slow-burning contest to be Labour’s next leader, however long it has left to run. In 2015, she gave her nomination to Jeremy Corbyn – her immediate neighbour as an Islington MP – when he ran for leadership. That was not because of his politics, but because of his kindness towards her. Other MPs tell the same story.

When the next contest comes, the candidate in pole position will have Corbyn’s personal likeability. Whether he or she will share his politics, however, is a more open question.