WHEN Theresa May called the general election eight weeks ago, Jeremy Corbyn was widely regarded as the weakest leader the Labour Party had fielded since Michael Foot in 1983 or perhaps even since George Lansbury in 1935. Today he is the comeback grandpa: a potential kingmaker in Parliament and the undisputed leader of the Labour Party.

There is a small chance that Mr Corbyn will be able to form a coalition government. As we went to press the Tories looked as if they could fall just short of an overall majority. By convention, the incumbent party has the first chance to form a government. Yet the largest of the minority parties, the Scottish National Party and the Liberal Democrats, would be far more likely to make a deal with Labour than with the Tories. Even if the Tories are able to form a government, Mr Corbyn will be the powerful leader of a powerful opposition: he will be able to put a prime minister who has at best a small majority under constant pressure.

Mr Corbyn has revolutionised the British left. Since the mid-1980s Labour has assumed that the only way that it can have any chance of winning power is to cleave to the centre: drop left-wing policies such as nationalising industries or supporting “national liberation struggles” and embrace the market and the Western alliance. Mr Corbyn was one of only a handful of MPs who held out against this argument. Tony Blair and his allies treated him as an irritant and an eccentric.

The vast majority of Labour MPs agreed with the Blair approach until only the other day. In 2015 he scraped onto the ballot to become the party’s leader, a contest he never expected to win. Last year three-quarters of Labour MPs voted against him staying on in the job, in a failed attempt at a coup. To many, Mr Corbyn looked like an occupying force, supported largely by a small group of faithful hard-leftists in his office, notably Seumas Milne, his chief strategist, and, outside Parliament, by Len McLuskey, the boss of the Unite trade union, and by Momentum, a grassroots pressure group of activists, many of them until recently members of other parties.

The parliamentary Labour Party will now be engaged in a fundamental rethink. Mr Corbyn has redefined the parameters of the possible. He has demonstrated that the Labour Party can do well by standing for what it “really” believes in, rather than cleaving to the centre. Ed Miliband, the previous leader, lost the 2015 election in part because he always seemed to be apologising for himself.

In contrast, Mr Corbyn has always been proud of his socialist record. He has also demonstrated that the tabloids are not the Rottweilers that Labour feared. The stories about Mr Corbyn’s associations with the Irish Republican Army have a lot of truth in them but large numbers of people, particularly the young, didn’t care. The Blair era truly ended on June 8th.

Mr Corbyn will face serious constraints. If he forms a coalition he will have to compromise with his partners and if he leads the opposition he will have to come to terms with the critics in his own party. Some Labour voices will also argue that a different leader might have actually won this election. After all, the economy is weakening, the public was weary of austerity, the National Health Service is experiencing repeated crises and, on top of all that, Theresa May fought the worst election campaign in recent history.

All this may be true—but it is a hard case to make stick. Mr Corbyn fought a strong campaign against all expectations. He may not have won the election but, unlike the leader of the Conservative Party, he now has the aura of a winner.