Khan went into the election with no seats in Pakistan's 342-strong parliament and few predicted that his party would win in more than a handful of constituencies.
A last-minute surge of support for Imran Khan's upstart political party has put Pakistan on a knife edge as the country prepares for Saturday's historic election.
The former cricketer's emotional final appeal from his hospital bed - where he is recovering from a fall that fractured his spine - to a crowd of 25,000 in Islamabad on Thursday night has shaken the camp of the front-runner Nawaz Sharif, who is campaigning for a third term as prime minister.
Privately, Sharif's supporters admit that his prospect of outright victory has slipped away, and that a coalition looms.
Khan went into the election with no seats in Pakistan's 342-strong parliament and few predicted that his party would win in more than a handful of constituencies. For him to stand even a faint chance of leading the largest group of MPs after today's vote is a remarkable transformation.
Tariq Azim, a former minister who is one of Sharif's key lieutenants in his PML-N party, insisted that Khan was still too far behind to cause such an upset. "We are confident of victory, but how big that will be is still to be seen," he said.
However Khan's campaign has electrified what might have been a predictable affair, fought between two established parties that have swapped power at regular intervals.
The race has been marred throughout by violence, including from Islamist groups which object in principle to Pakistan's democracy. Four people were killed yesterday when a bomb delivered by motorcycle was detonated in Miranshah, the main town of North Waziristan, in the tribal belt. The Pakistan Taliban threatened to attack polling stations today and hundreds of thousands of security officers were being deployed last night to ensure voter safety.
There have been no polls for the past two months, making prediction of the result even more uncertain. Sharif's supporters believe his party's rural network of influential supporters, able to mobilise thousands of voters, put it in pole position, but privately they admit a high turnout could help Khan's Movement for Justice party. "That will bring out the young and women, and we aren't so sure which way they'll go," said a party worker in its Punjab heartland.
In the meantime, doctors have taken away Khan's mobile telephone and banned visitors other than family to make sure he recovers in hospital, rather than leading the campaign from his bed. On Tuesday he fell 15ft from a forklift truck, which was raising him on to a platform where he was due to make a speech.
His campaign has been bolstered by the arrival of dozens of British Pakistanis. One or two hold key positions in his campaign hierarchy, while others are working in the party's energetic social media team. They have talked enthusiastically about learning from the way Barack Obama used new media to leapfrog more established candidates for the White House in 2008.
"While Khan was initially handicapped by the lack of party organisation and the absence of a formal presence at the provincial level, he managed to overcome these challenges by establishing a network of volunteers who have campaigned frenetically and held massive public rallies," said Shamila Chaudhary, senior editor at Eurasia Group.
Cyril Almeida, a columnist with Dawn, said Khan would most likely struggle against the PML-N's sophisticated election machine but that nothing was certain. "It was always on a knife edge so any surge by any one party can throw everything off," he said.
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