On 13 September 2015, for the first time in British history, a Marxist entered the office of shadow chancellor. Unlike Jeremy Corbyn, who recently confessed that he had not “read as much of Marx as I should have done”, John McDonnell is described by friends as a “true follower” of the philosopher.

Labour MPs have long suspected that his admiration extends to Lenin and Trotsky, the leaders of the 1917 Soviet revolution. A lengthy 2006 interview with the Trotskyist Alliance for Workers’ Liberty, unearthed by the New Statesman, confirmed their belief. Asked to name the “most significant” influences on his thought, McDonnell (who was then standing for the Labour leadership) replied: “The fundamental Marxist writers of Marx, Lenin and Trotsky, basically.”

Though the shadow chancellor has praised Marx since his appointment (“You can’t understand the capitalist system without reading Das Kapital”), he has unsurprisingly avoided any reference to Lenin or Trotsky. Unlike Marx, the Soviet duo were responsible for the mass murder of political opponents and inaugurated many of the communist state’s dictatorial methods. In Labour circles, they are reviled as the intellectual inspiration for the entryist Militant Tendency (expelled from the party under Neil Kinnock). But as recently as 2006, the shadow chancellor cited them as definitive influences on him.

Many will ask how a politician with such loyalties remained in the party throughout Tony Blair’s leadership. Still more will ask how he can now speak in social democratic terms of “transforming” capitalism, rather than abolishing it. It is McDonnell himself who offers the best explanation of these apparent contradictions. In the same 2006 interview he argued that the first-past-the-post electoral system and “the affinity and loyalty of the large section of the labour movement and of the working class” to Labour (“you can call it false consciousness or whatever”) meant for “very pragmatic reasons” it was “important to work within” the party.

To the charge that his leadership campaign was a “doomed effort”, he countered: “It's the old Gramsci thing - we're trying to win the battle of ideas on the basis of a really thorough, democratic debate. So we're trying to win hegemony within both the party and the country. And then, use that battle of ideas to make sure we can reflect that in the battle of organisation within the movement overall.”

Antonio Gramsci, one of the most influential Marxist thinkers, advocated a “long march through the institutions”. This entailed working within established organisations (such as Labour) with the intention of winning them for the revolutionary cause.

McDonnell’s strategy was derided by leftists such as George Galloway, who formed the rival Respect Party. But with the election of Corbyn as leader it was vindicated. In 2006, McDonnell spoke presciently of a “dormant” left which could “easily be re-engaged” with Labour. Ed Miliband’s reform of the party’s leadership voting system in 2014 transferred sovereignty to them. In what he regarded as the twilight of his political career, McDonnell’s “long march” finally bore fruit.

As shadow chancellor he has said that he both hopes and believes Labour will win the next general election. But some, such as former No.10 policy head Geoff Mulgan (who worked with McDonnell at the Greater London Council), have suggested that he may be pursuing a version of Trotsky’s “transitional programme”. According to this strategy, socialists should make economic demands they know are unachievable in the hope of stirring up greater discontent with the system. Previously unreported remarks by McDonnell suggest this interpretation is not outlandish. During a House of Commons debate on 4 July 2011, the then Labour backbencher said: “As someone who still sees the relevance of Trotsky’s transitional programme, I am attempting not to salvage capitalism but to expose its weaknesses.” More recently, McDonnell has suggested that he hopes to retain capitalism but in a more egalitarian form. The question remains as to whether this is merely a tactical shift or an ideological one.

With Corbyn as leader, the left is no longer what McDonnell called “an opposition within our own party”. But its rule is potentially threatened by Labour MPs, just 14 of whom (out of 230) voted for Corbyn. The leader’s opponents intend to launch a challenge to him after the EU referendum on 23 June. Though most acknowledge that they are likely be unsuccessful if Corbyn is automatically included on the ballot (a recent YouGov poll showed undiminished support among party members) they are prepared to make multiple attempts. “We only need to get lucky once, he needs to get lucky every time,” a shadow minister told me. Corbyn, they hope, will succumb to death by a thousand cuts.

In 2006, McDonnell cited “Trotsky’s analysis of the bureaucracy” to explain the creation of a “degenerate” Parliamentary Labour Party. “The leadership replaces the central committee, the central committee replaces the membership. And that's what's happened here. That coup has allowed the ruthless use of patronage to isolate the PLP from any democracy or accountability within the party itself.”

Among both supporters and opponents of Corbyn, McDonnell is increasingly spoken of as possible successor if his ally is ousted or resigns before 2020. “If Jeremy was pushed under a bus being driven by Boris Johnson, it would all rally behind John McDonnell,” Ken Livingstone told Russia Today last month.

McDonnell, who again stood unsuccessfully for the Labour leadership in 2010, has long been the senior partner in his relationship with Corbyn. Both friend and foe describe him as more competent, more experienced and more intellectually able. Shadow cabinet ministers say he is an “increasingly dominant” presence.

After the PLP meeting on 14 March 2016, a spokesman told journalists: “No one criticised John [McDonnell] or the fiscal credibility rule”. The shadow chancellor’s pledge to run a current budget surplus while borrowing for investment is regarded by most MPs as sound economics. But McDonnell’s recent admiration of Lenin and Trotsky will only confirm their fear that the messenger risks crowding out the message.