Scrapping the 50p top rate of income tax would be "morally repugnant" and "economically witless", Liberal Democrat president Tim Farron told his party's conference in Birmingham.
In a speech which made a point of highlighting the differences between the coalition parties, he said Liberal Democrat ministers were reining in the Conservatives over calls for the 50p tax rate for people earning over £150,000 to be axed.
"At a time when 90% of the country is struggling to pay the rent or the mortgage, giving a 10p tax cut to those who need it the least, would not just be economically witless, it would be morally repugnant.
"Danny (Alexander), Vince (Cable), Nick and I are absolutely clear - the wealthy will continue to pay the largest share of the cost of our recovery so that we can protect the least well off.
"The principle that the rich pay more does not come from a desire to penalise the wealthy, but from a desire to ensure that our recovery must be a fair recovery.
"Be absolutely sure of this. Liberal Democrat ministers are the guarantors of fairness in a government that would be an absolute nightmare without them."
Farron's comments came after deputy Prime Minister Nick Clegg said he could block any attempt to scrap the 50p top rate of tax.
On the eve of the conference, he said cutting rates for the wealthy while millions were struggling to cope could "destroy" public support for the entire tax system.
Clegg said the Lib Dems would back abolition of the 50p rate in the long run only if it was not raising much revenue and was replaced by new taxes on "unearned income". These could include a 1% annual "mansion tax" on homes worth more than £2m, a land tax and restricting tax relief on pensions to the basic 20p rate.
In an interview with The Independent, Clegg also acknowledged the Government had to do more to boost growth in the economy, adopting what he called a "Plan A-plus". Chancellor George Osborne has always insisted there was no "Plan B" alternative to his deficit reduction plan.
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