Jeremy Corbyn was once so disliked as the leader of Britain's Labour Party that most of his caucus refused to work with him and a vast majority of Labour MPs voted to push him out.

But now, the man seen by many as too left-wing, too boring and too out of touch to ever be elected prime minister is enjoying a remarkable renaissance thanks to the stunning results of last Thursday's election that saw Labour come close to defeating Theresa May's Conservative government. The outcome has left Ms. May scrambling to cobble together a minority government amid a growing clamour for her to resign. Meanwhile, the 68-year-old diehard socialist is readying for a final push to topple Ms. May and move into 10 Downing St.

"I can still be prime minister," Mr. Corbyn declared on Sunday. "This is still on. Absolutely."

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Mr. Corbyn not only led his party to increase its seat total by 32, but Labour also came within two percentage points of the Tories in the popular vote, overturning a 20-point Tory lead when Ms. May called the election in April. Now, as the leader of the biggest opposition party in the House of Commons, Mr. Corbyn will play a key role in determining just how long Ms. May's government survives. And he'll be instrumental in shaping every government initiative, including Brexit, where he has already said Ms. May's hard Brexit strategy lacks credibility. He prefers a softer Brexit, where the country keeps some ties with the European Union, such as remaining in the single market, which allows for the free flow of goods, services and people. That's in line with every other political leader in Parliament, as well as the popular sentiment. With EU negotiations set to begin on June 19, experts say Ms. May will have little choice but to follow suit.

As he made the rounds of television shows on Sunday, Mr. Corbyn was no longer the bumbling ideologue portrayed by many British newspapers for so long. Instead he had the confident air of someone who has turned the country's politics upside down. His next act will be to try to bring down Ms. May's government as soon as possible, he said, even as early as next week when the new government introduces its Throne Speech.

"We are quite ready and able to put forward a serious program which obviously has massive support in this country," he told the BBC.

Mr. Corbyn's rise is nothing short of spectacular considering how far he has come. He'd been a marginal character in the Labour Party for decades, stuck on the fringes of the party's far-left wing and a constant thorn in the side of Labour reformers such as Tony Blair. He has voted against his own party's dictates 500 times since becoming an MP in 1983 and he led the campaign against the war in Iraq, which Mr. Blair famously pursued with U.S. president George W. Bush. When the party leadership became open in 2015, Mr. Corbyn was supposed to be the token candidate to appease the party's activist wing. No one gave him a chance of winning. He shocked the party establishment by taking 60 per cent of the votes. But even then, most Labour MPs and the party brass refused to accept his leadership – viewing him as unelectable – and he faced a constant stream of challenges.

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When Ms. May called the snap election, the party hierarchy braced for a wipeout. At first, it looked as if the naysayers were right. Mr. Corbyn bungled the launch of the party's manifesto and senior MPs looked uncertain when it came to policy details. But once he finally laid out the party's platform, Mr. Corbyn caught fire.

His call to tax the rich, nationalize railways and provide free tuition for university students struck a chord in a country fed up with austerity and tired of rising inequality. His Bernie Sanders-like crusade took the country by storm and soon his rallies began overflowing with supporters, many under the age of 30 who embraced his message of change.

One rally toward the end of the campaign typified the Corbyn phenomenon. It was held in a community centre in Basildon, a Tory stronghold just east of London. Hundreds of people packed into a giant meeting room to hear him speak, many standing against the walls clutching handmade signs saying "JC for PM." Among those at the back was 18-year-old Andrew Gardner, a Tory supporter now backing Mr. Corbyn. "I definitely like his plan to drop tuition fees," he said standing next to his friend, Steven Eyres, also 18, who switched to Labour as well.

Not far away, long-time Labour supporter Freddy West marvelled at the size of the crowd. He couldn't believe Mr. Corbyn would come to a place such as Basildon, which hadn't voted Labour in years. But even he'd seen something unusual happening. Normally when Mr. West, 70, campaigns for the party in his riding, he gets brushed off or even harassed. But now people were coming up to him, asking for pamphlets and signing up to help. "Everybody is fired up," he said.

Outside the community centre, Ann Kobayashi waited more than an hour just to catch a glimpse of Mr. Corbyn leaving. She had quit the Labour Party under Mr. Blair but returned to the fold once Mr. Corbyn became leader, believing he was truer to Labour's roots. "The mainstream media were pretty bad about him," she said. "It's only in the last few weeks that he's been heard and seen and people are beginning to think, 'Where is this evil monster we've all heard about? Because he seems to be speaking sense.'"

While Mr. Corbyn won rave reviews for his campaigning, Ms. May floundered. She botched the launch of the Conservative manifesto and had to scrap a key reform to an elder-care benefit that quickly became dubbed a "dementia tax." Far from showing the country strong and stable leadership, Ms. May looked weak and wobbly.

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"What's surprised a lot of people is he's become quite a serious politician," said Simon Hix, a professor of European and comparative politics at the London School of Economics. "He's done his homework, he's prepared himself much better and he comes across as very sincere."

The results have thrown Ms. May's future in doubt and left her reaching out to the tiny Democratic Unionists for survival. But even that alliance will only yield her a cushion of two seats above the 326 majority in the House of Commons and it's far from certain the two parties are compatible. The DUP disagrees with just about everything Ms. May stands for on social policy and doesn't accept her hard Brexit position.

But Mr. Corbyn, too, faces pressure. Bringing down the Tory government as Britain and the EU begin Brexit negotiations may not be judged kindly by voters – nor would plunging the country into another election if Labour fails to form a government in the aftermath.

For now, he's basking in the glow of unexpected electoral success. When asked on Sunday if he would lead the party for the long term, Mr. Corbyn gave a wry smile and replied: "Look at me, I've got youth on my side."