Nick Clegg has torn into Labour's record on tax, on the day the governing party unveils its manifesto.
The Liberal Democrat leader said the Labour party had failed to deliver on their promises on "fairness and new politics" contained in the manifestoes for the 1997, 2001 and 2005 campaigns.
Speaking in an early morning press conference alongside Treasury spokesman Vince Cable, Mr Clegg said: "If they haven't managed to do it in 13 years, why on earth would anyone believe that they're going to be able to do it this time?
"We now have evidence that despite all the rhetoric from Labour, for 13 years now they've actually presided over a tax system which has seen an acceleration in equality, a decline in social mobility and is actually hammering people on low incomes, forcing them to pay more tax to the taxman than people at the top.
"What more evidence do you need that Gordon Brown and the Labour party have failed on fairness?"
Mr Clegg refused to rule out the possibility of a coalition between Labour and the Lib Dems in the scenario of a hung parliament, however.
He said that the party which has "demonstrably got the biggest mandate... has the moral right first to seek to govern, either on its own, or to reach out to other parties".
The Lib Dem leader downplayed the number of times a close result had been reached, before February 1974 - the last hung parliament - was mentioned.
He added: "I'm saying we're not trying to repeat history, we're not trying to draw lessons from history. What we're trying to establish is a principle - that there are no deals between politicians which prevent people having their say."
The Lib Dems unveiled evidence today that the poorest 20% of people are losing more of their income in tax than they did in 1997, while the richest 20% are paying less.
"Despite everything they said in 1997, life has got harder for people at the bottom and easier for people at the top," Mr Clegg added.
"All the promises of fairness in Labour's manifesto should be taken with a barrelful of salt. They cannot be trusted. This is yet another manifesto for an unfair Britain."
The Lib Dems complemented the attack with further details of the party's income tax plans, which officials say would take the lowest earners out of income tax altogether.
The £17 billion tax switch is said by the party to put an average of £700 into the pockets of millions of people on low and middle incomes by raising the personal allowance to £10,000.
The income tax cuts will be paid for by closing loopholes benefiting those at the upper incomes, a crackdown on tax avoidance, a new mansion tax on homes worth over £2 million and higher aviation duties.
The Lib Dem manifesto will be published in full on Wednesday.