Thirty-five years ago in January, a group of Labour MPs filed for ­divorce from their party. Their grounds included the left’s relentless advance, the planned deselection of colleagues and the adoption of unilateral nuclear disarmament as policy. After announcing their intention to secede on 25 January (in the Limehouse Declaration), Roy Jenkins, David Owen, Bill Rodgers and Shirley Williams – the “Gang of Four” – formed the Social Democratic Party on 26 March 1981. Twenty-six of their Labour colleagues joined the breakaway group. Explaining her decision, Williams wrote at the time: “The party I loved and worked for over so many years no longer exists.”

As they struggle to come to terms with Jeremy Corbyn’s election, many Labour MPs express similar sentiments. The most left-wing leader in the party’s history is remaking it in his own image. Labour, which inaugurated Britain’s nuclear weapons programme in 1946, now has an opponent of Trident, Emily Thornberry, as its shadow defence secretary and Corbyn, a lifelong unilateralist, is seeking to align the party’s position with his own. Before the House of Commons vote on Trident renewal, later this year, he may stage a ballot of Labour members. The expectation of his supporters and opponents alike is that they would endorse abolition.

In the nine months since the general election, Labour's membership has changed out of all recognition.Of the party’s 388,407 dues-payers, 187,114 have joined since May 2015 and 87,158 since Corbyn’s victory in September. They are overwhelmingly supportive of the leader. Those who voted for David Miliband rather than his leftish brother in 2010 are now a minority. In contrast, just 14 of Labour’s 231 MPs backed Corbyn.

When Corbyn achieved his landslide, some of his opponents optimistically hoped that his leadership would not last. There was talk of a “coup” before Christmas, and others asked whether the former backbencher would choose to spend more time with his allotment. Those who agreed to serve under him (dubbed the “making it work” group) believed they could act as a brake on the party’s leftward trajectory.

Four months later, all of these predictions have proved wrong. Not only has Corbyn survived, he has been strengthened. Labour’s emphatic victory in the Oldham West and Royton by-election on 3 December, which some MPs feared the party would lose, secured his position until at least May. The result emboldened him to carry out a shadow cabinet reshuffle in which he replaced the pro-Trident Maria Eagle with Thornberry and sacked the then shadow culture secretary, Michael Dugher, and Pat McFadden, the then shadow Europe minister. His ruthless assertion of patronage demonstrated that he would not tolerate what his aides called “disloyalty”. Corbyn is determined not merely to hold office, but to wield power, too.

An increasing number of people in Labour believe that he will still be leader in 2020 when the next general election is held. They see no evidence that he will resign voluntarily (“He’s enjoying it,” insiders often say) and expect him to retain the loyalty of a still-growing membership. Under Labour’s new one-member-one-vote system (which many on the party’s right had long advocated), it is the activists who are sovereign. Few believe that Corbyn could be denied a place on the ballot if challenged for the leadership.

His opponents speak balefully of the potential consequences of a Corbyn-led party contesting a general election in 2020, suggesting that Labour could fall below 200 MPs for the first time since 1935. Its present polling position is the worst of any Labour opposition since 1945 and it is forecast to become the first to lose council seats in a non-general-election year since 1985.

It is in this context that some MPs argue that if they cannot regain control of their party, Corbyn’s opponents must emulate the SDP and form another. Peter Hyman, a former strategist and speechwriter for Tony Blair, writes on page 29: “Politics today is far more fluid and people are crying out for a fresh voice in British politics. If, but only if, the Labour Party does not get back on track in the next 18 months, then there will need to be serious consideration given to a new party of the left emerging.” Assem Allam, a former Labour donor with an estimated worth of £320m, has pledged to fund a breakaway.

Labour MPs privately confess that they would take this route “if we thought it would work”. But they do not. The experience of the SDP-Liberal Alliance and, more recently, Ukip shows how first-past-the-post acts as a formidable barrier to “breaking the mould”. Of the 28 Labour MPs who defected to the SDP in 1981, as Corbyn’s ally Ken Livingstone reminded me, just six were re-elected.

Not only did the breakaway fail, but its parent party eventually succeeded. After 18 years in opposition, Labour won three successive elections for the first time in its history. It is from those who led the fightback in the 1980s – Neil Kinnock, Roy Hattersley, Giles Radice, John Smith – that Corbyn’s opponents draw inspiration, rather than “the splitters”. As long as Labour remains the main opposition party, it will attract support as the only force capable of removing the Tories from office.

Other considerations militate against a split. Labour has no figures of the stature of the Gang of Four (two of whom, Jenkins and Owen, had held great offices of state): its medium-sized beasts far outnumber its big ones.

The political territory for a new centre-left party to colonise is also narrower than in the 1980s. The Tories have privatised services left untouched by Margaret Thatcher and plan to cut public spending to one of its lowest levels since the Second World War. But they are also introducing what is among the highest minimum wages in the world and imposing an apprenticeship levy on business: a smaller but more interventionist state. They have established equal marriage and have widened their appeal to ethnic-minority voters. Polls show that voters regard David Cameron as significantly more centrist than Thatcher.

Corbyn’s team argues that those considering a split understate his mandate and overstate the degree of division. On economic and social issues, it says, the party is more united than it was in the past. Only on Trident is there an unbridgeable divide. The party leader has made compromises, committing to EU and Nato membership and abandoning the proposed nationalisation of the energy sector. Allies say that although Corbyn and others frequently rebelled under New Labour, Tony Blair and Gordon Brown never faced such irrevocable hostility (Corbyn was shouted down at a PLP meeting weeks after his victory). However, MPs reply that it was Blair’s electoral success that earned him loyalty. Long before Corbyn’s election they had resolved, after two successive defeats, that any leader who appeared unelectable would be removed before 2020. In the case of Corbyn, many have already concluded that Labour will only go backwards on his watch.

As they dismiss the possibility of a split, Labour MPs are contemplating alternatives. An option that is increasingly being discussed is the one proposed three weeks ago by Joe Haines, Harold Wilson’s former press secretary, in the New Statesman. Haines called for MPs to assert their mandate from Labour’s 9.3 million voters and elect their own leader. “In the end, the parliamentary party determines its own standing orders and its own affairs,” a former shadow cabinet minister told me. “We have the ability to choose who is the leader of the opposition. It would essentially be a split between Labour members, or part of the membership, and Labour voters.”

Others predict that a mass resignation of frontbenchers, perhaps combined with a trade union revolt, could force Corbyn to stand down. Most do not expect a coup attempt this year. The likely victory of Sadiq Khan in the London mayoral election will, MPs believe, distract from parlous results elsewhere in May. After just seven months as leader, Corbyn will also be able to plead for more time. The probability of an EU referendum this year further limits MPs’ room for manoeuvre.

It is in 2017-18, if Labour’s position does not notably improve, that many believe a reckoning will come. Senior figures from all wings of the party have already fired warning shots. Len McCluskey, the general secretary of Unite, Labour’s largest donor, has said that Corbyn has “two or three years” to develop an appealing alternative to austerity. The former party leader Neil Kinnock (interviewed on page 24) told me that it was “difficult to see” Corbyn as electable and that if he was “seen to be failing to connect to the electorate after a reasonable space of time, then he may come to his own conclusions”. He added: “Those people who want to win power, whether they’re left, right or centre, will be watching the evidence and will make their decision on the basis of that evidence.”

In contrast to the regicidal Conservatives and Liberal Democrats, Labour is tolerant of floundering leaders. But those who anguished over whether to remove Brown and Ed Miliband, partly out of loyalty and partly out of fear of the consequences, will have no such qualms about challenging Corbyn. Rather than the SDP’s example, it is the advice of Margaret Thatcher that MPs are more likely to follow. “They should have stayed within and fought their way through,” she remarked of the Gang of Four during the 1983 general election. “The Labour Party won’t die. The Labour Party will never die. They should have fought.”