The mood at the first shadow cabinet meeting of the year on 12 January was sombre. “We can’t go on like this,” Andy Burnham told colleagues, warning that the voters would never support a party as divided as Labour. To the surprise of some present, John McDonnell, the shadow chancellor and Jeremy Corbyn’s closest ally, agreed, vowing to “draw a line” under recent conflicts. All sides in Labour recognise that unity, a quality revered by the wider labour movement, is desirable. The question that divides them is: “On whose terms?” Corbyn’s recent reshuffle further polarised a party that is more profoundly split than at any time since the 1980s.

Rather than healing the wounds caused by the sacking of the former shadow culture secretary Michael Dugher and the former shadow Europe minister Pat McFadden, Corbyn’s allies deepened them. Diane Abbott derided four of the departed frontbenchers as “former special advisers” (mistakenly including Jonathan Reynolds), as if that in some way tainted them. McDonnell denounced them as part of a “narrow right-wing clique”.

There was derision at the Parliamentary Labour Party (PLP) meeting on 11 January when the shadow chancellor initially failed to turn up. He arrived 40 minutes late, after some MPs had left. One called him a “spineless bastard” for not having “the balls to turn up” after “slagging off” colleagues.

Earlier that day, the shadow attorney general, Catherine McKinnell, became the first shadow cabinet member to resign under Corbyn, citing family reasons but also the party’s “increasingly negative path”. McKinnell, who voted for Ed Miliband in 2010 and for Yvette Cooper last year, is “far from a ‘Blairite’”, one senior MP noted.

When they chose to serve under Corbyn, many shadow cabinet members did so out of more than team loyalty. Though doubting his electability, they hoped that he would forge unity over economic policy and ameliorate divisions over defence and foreign affairs. But the Paris attacks and the consequent air strikes against Isis in Syria made foreign policy a defining issue once more, intensifying the potential for division. The Labour leader conceded a free vote on intervention but such was the tortuous process by which he did so that many of his MPs regarded it as free in name only.

Still more divisive than Syria, over which the majority of the shadow cabinet sided with Corbyn, is the issue of the Trident nuclear weapons programme. The Labour leader could, some say, have set aside his unilateralism in favour of advocating aggressive multilateral disarmament. Instead, he chose to argue for full abolition, a stance shared by just six of his 29 shadow cabinet colleagues.

Corbyn reasonably contends that he has a mandate to do so, having been elected on a unilateralist platform, but MPs assert with increasing confidence that they have superior legitimacy, courtesy of the 9.3 million people who voted Labour in 2015. (The article in last week’s New Statesman by Joe Haines, Harold Wilson’s former press secretary, which thundered to this effect, has been studied and widely circulated.)

Burnham has pledged to resign if Labour endorses Trident abolition and two other members of the shadow cabinet say that they would follow. Unite and the GMB union, which represent workers in the defence industry, are also opposed to Corbyn’s position. The Labour leader has the support of many party members but unless he is able to reform the current policymaking process (which would require the approval of 60 per cent of trade union and constituency delegates), the unions will block any change in stance at the annual conference. In view of this – and with Trident renewal a fait accompli because of the majority support for it in the Commons – many ask why Corbyn is determined to have the fight. The answer is that, for him, it is a question of moral conviction. But this only confirms the chasm between Corbyn’s thinking and the realpolitik of his colleagues.

In advance of the May elections, the hope is that divisions will diminish as Labour faces the enemy without, rather than the enemy within. But the goodwill that Corbyn earned from his landslide victory in September has been progressively eroded. In these circumstances, would-be successors are making themselves known.

Dan Jarvis, the former paratrooper, has said that he regrets not giving “more thought” to standing in 2015. This week (on page 19), he explains what he calls his “radicalism and realism”. Owen Smith, the shadow work and pensions secretary, spoken of by insiders as a possible soft-left candidate, told me in a recent interview for the NS that it would be an “incredible honour and privilege” to lead the party. The shadow foreign secretary, Hilary Benn, the deputy leader, Tom Watson (whose supporters span the old right and the new left), the former shadow business secretary Chuka Umunna, the shadow energy secretary, Lisa Nandy, and the shadow energy minister Clive Lewis, a potential inheritor of the Corbynite flame, are the other names mentioned by MPs.

With few MPs confident that they will succeed under their present leader, a commonly expressed hope is that the Conservatives will fail. A schism over Europe or an economic crash, they suggest, could transform the terms of debate. Others fear that, as with John Major’s ascent in 1990, a change of leadership will settle a restive electorate.

The energy absorbed by internal conflict leaves Labour ill-equipped to challenge the Conservative Party as it ruthlessly and relentlessly consolidates its advantage. In what one shadow cabinet minister characterises as a war of “the weak against the weak”, neither Corbyn nor his opponents may be strong enough to triumph, leaving Labour stumbling towards 2020 with the conviction of a pantomime horse. The only unity in sight, the most despondent warn, is that of the dead.

Now listen to George Eaton discussing this subject on the New Statesman podcast: