There is a "right-wing threat to Britain" from the prospect of David Cameron being "held to ransom" by Ukip, the DUP and his own backbenchers, Nick Clegg has warned.

The Liberal Democrat leader said if the Tory party attempted to govern without an outright majority in the Commons, Mr Cameron would be dragged to the right by an alliance of MPs determined to slash public services and roll back human rights legislation.

Mr Clegg, who dubbed the alliance "Blukip", said that only the Lib Dems could offer a centre-ground alternative in the event of a hung parliament.

In a speech in Cheadle, a Lib Dem-held marginal where the Tories are mounting a strong challenge, Mr Clegg set out his party's pitch to moderate swing voters.

He said: "Everyone knows that David Cameron and Ed Miliband won't win this general election outright.

"That means that someone else will hold the balance of power on May 8.

"The prospect of a left wing alliance between Labour and the SNP has been well covered.

"What's equally possible is an alliance that will drag Britain further and further to the right: Blukip - a bloc of right wingers from Ukip, the Conservatives and the DUP that could hold the balance of power.

"There is a very real danger that Nigel Farage and his friends could hold David Cameron to ransom.

"It is a right wing alliance that brings together people who don't believe in climate change; who reject gay rights; who want the death penalty back and people who want to scrap human rights legislation and privatise our schools and hospitals."

He continued: "Do you want Liberal Democrats to keep the Government in the centre or Nigel Farage and his friends forcing David Cameron to lurch to the right?

"Do you want a coalition with a conscience or a coalition with a grievance?

"There is a right wing threat to Britain.

"The only way to stop it is to make sure there are enough Liberal Democrat MPs to keep the next government anchored in the centre ground."

The Lib Dems have published a list of 20 seats - five held by the Tories and 15 by Mr Clegg's party - that they claim could determine whether "Blukip" holds the balance of power.

The party unveiled a new poster showing Ukip's Mr Farage and Mr Cameron walking into Number 10, intended as a warning about the Blukip alliance.

Mr Clegg highlighted the views of MPs who could form part of the grouping as he launched his strategy, which is aimed at countering the Tory message that a vote for a party other than the Conservatives risks Ed Miliband taking the keys to Number 10 supported by the SNP.

Mr Clegg said: "Listen to Conservative MP Edward Leigh, who described belief in climate change as a 'thoroughly painful ideology' - one of a number of outspoken climate sceptics on the Tory and DUP benches.

"Listen to the DUP's Ian Paisley Jr, who said: 'I am pretty repulsed by gay and lesbianism'.

"Or his leader Nigel Dodds, who described the passing of equal marriage legislation as 'a source of great shame'."

He highlighted comments by other MPs arguing for schools to be run for profit, an insurance-based NHS and the Human Rights Act to be scrapped.

"These are just some of the Conservative, Ukip and DUP politicians who share a backward-looking right wing agenda," he said.

"With a hung parliament practically inevitable, there is a very real prospect that David Cameron may rely on these people to stay in No 10.

"You have all heard the mood music: David Cameron calling for Ukip supporters to come home to the Conservatives.

"Nigel Farage saying he would be prepared to prop up a Conservative government - and that more Tory MPs were preparing to follow Douglas Carswell and Mark Reckless in defecting to his party."

Mr Clegg said the Tory right had become "increasingly hardline" over the course of the last five years.

He added: "The Tory right wing has been waiting for its moment to reclaim the Conservative Party for years.

"And, with the help of MPs from Ukip and the DUP, that moment may be about to come."

Predicting the impact the right could have, he said: "Imagine what Britain could become if the Prime Minister has to bargain with Nigel Farage and his friends for votes.

"Our public services cut to the bone. Our communities divided. Our shared British values of decency, tolerance and generosity cast aside.

"Instead of Liberal Democrats holding the balance of power and using it to keep the Government in the liberal centre ground, Nigel Farage and his friends in the Conservatives and the DUP would drag Britain further and further to the right."

The "Blukip" strategy in Lib Dem-Tory marginals is one of several different campaigns being run by the party in the "micro contests" as part of the overall general election.

Party strategists predict that 2015 will "absolutely" see more tactical voting than previous contests, particularly in Scotland where they expect voters opposed to independence to swing behind the candidates most likely to block an SNP MP.

Mr Clegg insisted he was not exaggerating the potential influence of "Blukip".

He said: "Just listen to what the Conservatives say themselves. This is the whole point - I'm focusing on what the Conservatives are saying in seats like this, between the Liberal Democrats and the Conservatives.

"So if you have more Conservatives and fewer Liberal Democrats, and more DUP and more Ukip MPs, then this becomes more likely.

"Each Conservative that is elected in seats that are being contested by the Liberal Democrats makes it more likely this is going to happen. Just do the maths ... that's the inevitable consequence.

"Conversely, the better we do, the more unlikely it is that this will come about."

Mr Clegg, who referred to Mr Farage as "that man", said centre-ground Conservatives needed to be warned about the danger of "sleepwalking" to a hard-right government.

Asked if he would be prepared to share power with the Conservatives in a coalition which relied on Ukip's support, he said: "I don't want to be part of any arrangement where we have to dance to the tune of that man, Nigel Farage.

"I don't want to be part of an arrangement where basically the governance of our country is held hostage by people with extreme and hardline views."

The 20 seats listed by the Liberal Democrats as being the key to stopping a Tory-led government supported by Ukip and the DUP were: St Ives, St Austell and Newquay, North Cornwall, North Devon, Torbay, Cheadle, Hazel Grove, Berwick-upon-Tweed, Solihull, St Albans, Bath, Montgomeryshire, Maidstone, Chippenham, Berwickshire, Watford, Mid Dorset and North Poole, Taunton Deane, Somerton and Frome, and Oxford West and Abingdon.

Mr Clegg said: "If you are a voter in one of those 20 seats which are being contested between the Liberal Democrats and the Conservatives, the Conservatives will tell you 'vote Conservative to keep Ed Miliband out'. It's a nonsense, because the Conservatives are not going to win a majority anyway.

"But the real threat, the one which I don't think has been considered nearly enough, is this threat - is of a right-wing alliance cobbled together between Ukip, the DUP and the Conservatives.

"We are the only party that stands between the country and that future. That is why, instead of speculating on ways in which Nigel Farage provides back door rather than front door influence in government, I would much rather make sure that the government of our country remains firmly rooted in the centre ground."

Mr Clegg dismissed as "nonsense" an opinion poll which suggested that Lib Dem support had collapsed in the South West, where 14 battleground seats with the Tories lie.

He said: "We are going to do much, much better than this cottage industry of pessimism suggests. But I accept of course there's a real contest in those seats.

"The Conservatives are going round those seats saying 'you have got to vote for the Conservative candidate to keep Alex Salmond and Ed Miliband out of Number 10'. What they are not telling them is if you vote for the Conservatives you put that man (Mr Farage) into Number 10.

"That's the point about this event today. I want to raise the flag of alarm about a real, genuine prospect of a divisive, hardline, right-wing alliance governing our country after May 7.

"I don't want decent, centre-ground Conservative voters in those seats to sleepwalk towards that kind of future."