The Lib Dem leader dismissed a Comres poll as "bogus science" and said his party would win "many more seats" than predicted.
He also warned of a "hard-line right-wing" government - if Lib Dem seats are lost to the Conservatives, who would then be reliant on UKIP and DUP votes.
The Conservatives and Labour have both insisted they can win on their own.
But Mr Clegg said "everyone knows" that they will not and "that means that someone else will hold the balance of power on May 8" - be it a "left-wing alliance between Labour and the SNP" or a "right wing alliance" between the Conservatives, UKIP and the DUP.
The Lib Dem leader is campaigning in north-west England on Thursday. He and David Cameron are not taking part in tonight's BBC TV debate involving the leaders of other parties.
Asked how he stayed positive after a poll suggested his party could be wiped out in south-west England, losing all of its 14 seats in the region to the Conservatives, Mr Clegg said the poll itself was flawed.
The Comres poll of 1,005 adults across the 14 seats suggested a 13-point swing from the Lib Dems to the Conservatives, with Labour, UKIP and the Greens also making gains at the expense of Mr Clegg's party.
But the Lib Dem leader told LBC radio the poll had questioned the equivalent of 72 people per seat "without even mentioning the candidates' names".
"Polls like that are just absolute baloney," he said.
"The bogus science from some of this stuff is just extraordinary."
He added: "We will win so many more seats than this endless cottage industry of pessimistic punditry allows for."
His party says there are 20 marginal seats where a Conservative win, at the expense of the Liberal Democrats, would result in a minority Conservative government reliant on UKIP and DUP votes to stay in power "an alliance that will drag Britain further and further to the right". The Lib Dems have dubbed the alliance "Blukip".
The seats are; St Ives, St Austell and Newquay, North Cornwall, North Devon, Torbay, Cheadle, Hazel Grove, Berwick-upon-Tweed, Solihull, Brecon and Radnorshire, Bath, Montgomeryshire, Maidstone, Chippenham, Berwickshire, Watford, Mid Dorset and North Poole, Taunton, Somerton and Frome and Oxford West and Abingdon.
At a news conference in Cheadle, Mr Clegg said that would mean an alliance involving people who did not believe in climate change, wanted to bring back the death penalty and who were against gay rights: "There's a real prospect that David Cameron may have to rely on those people to stay in Number 10."
Mr Clegg said he wanted to "lift the lid" on a "genuine threat to our country" - because he said moderate Conservative voters in marginal seats were being told if they did not vote Conservative, they would end up with a Labour administration propped up by the SNP.
He said "moderate, centre-ground" voters did not want David Cameron being "held hostage" by the "swivel-eyed brigade" on the Conservative backbenches who had been "waiting for its moment to reclaim the Conservative Party for years".

Dundee-based artists Matthew Dalziel and Louise Scullion have created the art called Nomadic Boulders.
They were commissioned as part of a project involving Highlands and Islands Enterprise to revamp John O'Groats.
HIE said the art had to be designed to withstand the forces of 100mph winds that sometimes blow through the area.
Stones believed to be 25cm to up to 100cm in diameter are moved by fierce tides in the Pentland Firth.
Nomadic Boulders uses boulders which appeared on a neighbouring beach after a storm.

Councillors voted in favour of the Local Development Plan by 21 votes to five.
Protestors who feared the plan would negatively affect the numbers of Welsh speakers in the area had met outside the council offices in Llangefni.
Gwynedd councillors approved the plan on Friday.
Anglesey and Gwynedd had prepared a joint land use development strategy.
It includes some houses which have already been built or have planning permission.
Nick Clegg says reports that his party could lose 14 south-west England seats to the Conservatives are "baloney".
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New artwork inspired by large rocks that roll around the seabed of the Pentland Firth have been installed at John O'Groats.
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A plan for 8,000 new houses in Anglesey and Gwynedd over the next 15 years has been approved by councillors on Anglesey.