Nick Clegg has defended his claim there will need to be "savage" spending cuts and said politicians have to treat voters "like grown ups".
The Lib Dem leader accused Labour and the Tories of indulging in a "childish game" over the use of the "c-word".
He told the BBC it was "right to be clear about the enormity of the problem" and using "softer" language would not make it go away.
Gordon Brown talked of future cuts in spending for the first time last week.
Labour and the Conservatives have been locked in a row about public spending. The Tories have claimed the government is covering up plans for cuts while Business Secretary Lord Mandelson has accused the Tories of planning "savage" cuts.
'Softer language'
But Mr Clegg used the word himself in a speech on the opening day of his party's annual conference in Bournemouth, saying: "I have said there will need to be cuts, cuts that are savage and bold.
"But we will make those cuts so that we can be equally fierce - equally savage - about protecting the services that matter most, just as we put the nation's finances back in order."
Former Lib Dem leader Charles Kennedy has warned that the party is in danger of losing its "heart" over planned cuts and said party members were concerned when they heard "words like savage being bandied about".
Later he told BBC 2's Daily Politics: "I don't like brutal language in politics full stop, I think it repels people, so I think 'savage' has now given way to 'serious', I think is now the terminology that's being used, I'm more comfortable with that."
Mr Clegg told BBC Radio 4's Today programme he stuck by the word: "You can of course use softer, more emollient language, but that won't make the problem go away.
"We need to make significant, big, serious cuts in public spending if we are not only to fix the great black hole in the public finances but also to carryon on doing the thing we want to do as a society - looking after the frail, the elderly, the young."
He added: "Clearly I don't talk about the gravity of the situation ... with any relish but I want to do so with candour because I think it is wrong, simply wrong, for the Conservatives and Labour politicians to have this childish debate about whether they use the c-word or not - whether it's 8%, 9%, 10% for this department or that department."
"I have used the word savage and I think it's right to be clear about the enormity of the problem - but it's cuts for a purpose."
Trident renewal
The Conservatives had been accusing Gordon Brown of a lack of "candour" in not admitting to the need for cuts, during the long-running public spending row.
Last week the prime minister told the TUC conference he would "cut costs, cut inefficiencies, cut unnecessary programmes and cut lower priority budgets" but would "not support cuts in the vital front-line services".
Tory leader David Cameron has said his party would make cuts if it wins power, to tackle the budget deficit - which is expected to reach £175bn by the end of the financial year - although he has pledged to protect NHS spending.
The Lib Dems are the only party to say they would axe plans to renew the nuclear Trident missile system to save money.
The party's Treasury spokesman Vince Cable has also outlined proposals to scrap other big defence projects, like Tranche 3 of the Eurofighter aircraft, as well as reforming public sector pensions, scrapping ID cards and the NHS computer project.
But last week he accepted they would only save £14bn a year when what was needed was between £80bn to £100bn. He told the BBC: "It is eventually going to be a combination of spending and tax measures."
Mr Clegg also faces a backlash from party members after suggesting the pledge to scrap university tuition fees - at an estimated cost of £12.5bn - was not affordable in the current economic climate.
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