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 deputy prime minister 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.
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, 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."
Clegg said: "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 Clegg's party - that they claim could determine whether "Blukip" holds the balance of power.
1. St Ives
2. St Austell and Newquay
3. North Cornwall
4. North Devon
5. Torbay
6. Cheadle
7. Hazel Grove
8. Berwick-upon-Tweed
9. Solihull
10. St Albans
11. Bath
12. Montgomeryshire
13. Maidstone
14. Chippenham
15. Berwickshire
16. Watford
17. Mid Dorset and North Poole
18. Taunton
19. Somerton and Frome
20. Oxford West and Abingdon
