Liberal Democrat leader Nick Clegg says a vote for his party at the coming general election is "a bold commitment to hope and opportunity".
He told the party's spring conference that the election, widely expected on 6 May, was "a once in a generation opportunity for real change".
Mr Clegg insisted a vote for his party would not be a wasted vote.
But ministers said the next government would be Labour or Conservative. The Tories claimed Mr Clegg was "confused".
The BBC's political correspondent Mike Sergeant said Mr Clegg used his speech to activists in Birmingham to set out the big themes of his election campaign, discussing his plans to reform tax, education, the economy and Parliament.
He accused Labour of being the party of "many disasters" and said the government lacked the "courage and honesty" to take tough action to reduce the UK's huge fiscal deficit.
I agree with Clegg. Labour and Conservative have been the only two parties in government for years. Not only would the Liberal Democrats bring change, they would bring positive change, such as fairer taxes and more open politics.
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