He urged voters to put the Lib Dems in office beside David Cameron or Ed Miliband, with the polls suggesting neither one is likely to win a majority.
However Lib Dem Minister for Pensions, Steve Webb, a Christian, told Premier's News Hour the party wasn't prepared to discuss pre-election pacts with any one party before May 7: "We don't go into the election saying we prefer these people or those people.
"What we do today is say what we do stand for, and the more of us we're able to get into parliament, the more of that manifesto on a fairer society and a stonger economy we can get delieved," he added.
Mr Clegg said only his party could "add a heart to the Conservatives, and a brain to Labour" and only the Lib Dems could help guarantee the right path between the excessive cuts of the Tories and the excessive borrowing of Mr Miliband's party.
Meanwhile, Nigel Farage says his party's policies are "serious, fully costed", and for people who "believe in Britain".
Launching their election manifesto in Essex, the leader promised Ukip would increase defence spending, and provide jobs for Army veterans.
Mr Farage also wants an Australian-style points-based system for migrants coming to the UK, and an in-out EU referendum "as soon as possible".
Lib Dem Minister for Pensions, Steve Webb speaking to Premier's Des Busteed on the News Hour:
