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People believe that "something exciting" is starting to happen in politics, said Nick Clegg.He was speaking to journalists on a visit to a hospital in south west London after a poll indicated a boost for his party after the TV debate.
Mr Clegg said a growing number of people were "starting to hope that real change" was possible in Britain.
But David Cameron warned of a hung parliament, seen as more likely if Lib Dem support increases.
Mr Clegg's comments came as his party's policies came under close scrutiny with Labour criticising their views on Trident.
'Old politics'
The Liberal Democrat leader, on his visit to Kingston Hospital, said: "I think change, real change is more possible than people think when a growing number of people start thinking, believing, that maybe we can do things differently.
"I think that is what is starting. I can't predict what is going to happen in the election campaign, but I think something exciting is starting to happen."
Asked if by "something different" he meant a formal coalition, he said: "I don't think we should put the cart before the horse."
But he said that more people were turning away from the "old politics" of Labour and the Conservatives.
"If a growing number of people do that then the possibilities of doing things differently really are huge."
"There's a point where suddenly a critical number of people say suddenly 'maybe I'm not going to vote like I did last time, maybe I don't have to vote the way that my grandparents did, the way that my street, my community always have done'," he continued.
Mr Clegg was joined by his wife Miriam on the visit to the hospital where she gave birth to their son last year. They chatted to staff and parents.
He said all politicians would promise to protect A&E departments, maternity wards and GP services.
"The difficult thing, which is why we keep banging on about this, is how we do that," he said.
He pledged to scrap strategic health authorities and computer databases to reduce the cost of NHS administration.
Meanwhile Mr Cameron said the way to get things done was to have a decisive Tory government, not a hung parliament.
Mr Brown campaigning in Bedford and Milton Keynes made no mention of the Liberal Democrats.
But earlier Defence Secretary Bob Ainsworth criticised the Lib Dem's policy on Trident as "ridiculous".
The Lib Dems "rule out like-for-like" replacement, instead saying they will seek "alternatives" and support efforts at multilateral nuclear disarmament.This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.