Nick Clegg insisted he was a serious challenger for No. 10 today as he declared: "I want to be Prime Minister."
The Liberal Democrat leader admitted many people do not regard his party as genuine contenders for power - but urged them to think again.
His aides boldly claimed their private research suggested they could yet overtake both Labour and the Tories to win the next general election.
They currently have less than 10% of seats in the Commons.
"I know there are people who agree with a lot of what we've got to say but who still don't vote Liberal Democrat," he told the party's annual conference.
"You don't think we're contenders. I urge you to think again."
In the closing speech to the Bournemouth gathering, Mr Clegg deliberately ducked coalition questions as he insisted he was going after power.
But he has also steadfastly refused to rule out an alliance in the event of a hung Parliament.
Aides to the leader insisted their polling suggested so many voters were undecided that the next election was wide open.
The Lib Dems have 63 MPs. They would need more than 300 seats to hold a majority in the House of Commons.
Recent polling has usually put the Lib Dems on a vote share of less than 20%.
That figure suggests their only chance of power is to team up with one of the other parties as the junior partner in a coalition.
But, towards the beginning of a 48-minute speech, Mr Clegg said he wanted the top job for himself.
"I want to be Prime Minister because I have spent half a lifetime imagining a better society, and I want to spend the next half making it happen," he said.
He issued a naked appeal to disenchanted Labour voters to switch their allegiance to his party and insisted a Tory administration was not inevitable.
"Labour is dying on its feet. We are replacing them as the dominant force of progressive politics," he said.
"We are the alternative to a hollow Conservative Party that offers just an illusion of change."
He urged voters not to turn to the Tories on the basis that "it's the only option".
In a grab for natural Labour voters, he hardened his position on Afghanistan - warning that "time is running out" - and restated the need for highly-redistributive tax reforms.
He added: "If you supported Labour in 1997 because you wanted fairness, you wanted young people to flourish, you wanted political reform, you wanted the environment protected or you simply believed in a better future, turn to the Liberal Democrats."
Mr Clegg also sought to shrug off criticism of his strategy after rowing broke out among members of his own frontbench.
He has come under fire for his strategy of "savage" spending cuts and watering down the party's commitment to abolishing tuition fees.
He did not address tuition fees directly but admitted it had been "quite a week" for the party.
"I am never going to duck asking the important questions however difficult they are," he told them bluntly.
"But I am immensely proud to lead a party that actually debates things, openly and democratically."
He warned there was "no easy solution" on public spending given the dire state of the public finances and acknowledged there would be "difficult" decisions to be taken.
But he insisted: "Taking them is the price of fairness."
After dismaying students over tuition fees this week, Mr Clegg made a play for the younger vote with a new promise to find them work or training within 90 days if they lost their job.
That would be funded by money raised from putting VAT back up to 17.5%.
He restated his pledge to raise the starting threshold for income tax to £10,000 and warned the rich and polluters that they would foot the bill.
"I'll be honest - if you've got a house worth over a million pounds, if you fly trans-Atlantic a couple of times a month, if you get a seven-figure bonus paid in share options to get around income tax, you will pay more.
"That is what is fair."
Mr Clegg also sought to appeal to voters who backed the party for its opposition to the Iraq War by hardening his language on Afghanistan.
He again urged Prime Minister Gordon Brown to take a different course in the conflict, calling for development assistance to be stepped up, talks with the Taliban and a new co-ordinated international strategy.
"We should do this properly or we shouldn't do it at all," he said.
"So I say to the Prime Minister - time is running out.
"Unless you change course, there will be no choice but to withdraw, and that would be a betrayal of the servicemen and women who have already made such enormous sacrifices on our behalf."Reuse content