Speaking to supporters in Cardiff on Monday evening, Mr Kennedy set out his vision for a future rooted in freedom, with better-funded public services and more money for pensioners.
" We will be that powerful voice that simply cannot be ignored "
He repeated pledges to abolish charges for eye and dental checks and scrap student tuition fees.
And he accused the Tories of proposing "totally irresponsible tax cuts", while dismissing the Scottish Nationalists and Plaid Cymru as "the ultimate wasted vote".
Addressing the wider electorate, Mr Kennedy said there were three "stark choices" facing voters on 7 June.
"If you think that the government has done enough for health, for education, for pensioners over the past four years then, yes, you should probably go out and vote Labour," he said.
"If you think the government should be doing even less to fund these services and making the priority income tax cuts, then without doubt you should go out and vote Conservative.
"But if you think there needs to be more investment in the health service, in education, that we can do more for pensioners, then your vote has got to be cast for Liberal Democrats at this election."
If Labour wins again there would have to be "effective and responsible opposition", Mr Kennedy said.
"Responsible opposition is about being prepared to work with the government when it is in the national interest.
"But effective opposition is also about opposing the government of the day with vigour where we disagree."
William Hague's Conservatives would be "utterly incapable" of providing this, he predicted.
And he declared the Liberal Democrats would become the real opposition in the next parliament.
"We will be that powerful voice that simply cannot be ignored," he said.
Kicking off the final week of the general election campaign, the Lib Dem leader had earlier repeated his argument for greater investment in public services.
" British people aren't stupid - they know you can't get something for nothing "
Part of the challenge for Liberal Democrats was to scrutinise "rather vague" themes touted by Prime Minister Tony Blair during the course of the election, said Mr Kennedy.
"Schools, hospitals, pensions and the police all require more resources.
"That has been the honest refrain of the Liberal Democrats' campaign over this past month and I think it's struck home to people.
"British people aren't stupid. They know you can't get something for nothing and that's why the Liberal Democrats have been honest with people over taxation during the course of this campaign," he said.
The Lib Dems plan to add a penny to the basic rate of income tax and to raise the top rate of tax to 50% for people earning over £100,000.