Nigel Farage has vowed to quit as Ukip leader if he fails to become an MP at the general election, declaring it would be "curtains for me".
Research has indicated he is on course for victory in South Thanet but the MEP will have to overturn a Conservative majority of nearly 17%.
Failing to secure the Kent seat would have "significant" consequences for both him and his party, he said.
The eurosceptic admitted it would be "just not credible" for him to stay at the top of the party if he failed on May 7.
In The Purple Revolution, serialised the Daily Telegraph, he wrote: "The consequences of me failing to secure a seat for myself in the Commons would be significant for both myself and the party.
"It is frankly just not credible for me to continue to lead the party without a Westminster seat."
"What credibility would Ukip have in the Commons if others had to enunciate party policy in Parliament and the party leader was only allowed in as a guest?
"Was I supposed to brief Ukip policy from the Westminster Arms? No - if I fail to win South Thanet, it is curtains for me. I will have to step down."
Mr Farage had to delay the start of his campaigning in South Thanet after the surprise defections of former Tories Douglas Carswell and Mark Reckless, which sparked by-elections in Clacton and Rochester and Strood.
He wrote: "I cannot win this seat on my own.
"But I think I can win it if I can show that I am part of a big, strong, active local team to support them, that is enthusiastic and capable."
He added: "While we are trying to keep well away from negative campaigning, the flak against me is quite substantial and I suspect it will only get worse the closer we get to the election.
"The other parties are obsessed with me not winning a Westminster seat and I can only hope that the hits on me will take the heat off the other Ukip target seats."
Mr Farage also accused his rival Al Murray, running in South Thanet in his comedy guise as the Pub Landlord for the newly-formed Free United Kingdom Party (FUKP), of "patronising" voters.
He told the Daily Star: "Clearly it's all been a bit of a stunt and we understand that. I think the attitude and his comedy act will be taken by a lot of people in this part of the world as slightly patronising.
"He's really laughing at them and I think the joke is starting to wear a bit thin. I don't think the failure to parachute is the relevant bit. His failure to answer the question as to why he is here and what he is doing is more important.
"I very much doubt he will be on the ballot paper."
Earlier this week, the Oxford-educated comic was told he was too heavy to parachute into Kent for a publicity stunt to promote his campaign.