Vince Cable has delivered a stinging attack on "lying" Tories who suggest the Government can balance its books without raising taxes.
Dismissing his coalition partners as "Ukip without the beer", the Business Secretary said they were "ideologically" committed to destroying public services and the welfare state.
He also called for more government borrowing to invest in major housing and infrastructure projects.
The intervention, in a keynote speech to the Liberal Democrat conference in Glasgow, came as the party fights to differentiate itself ahead of the general election.
Signalling a more aggressive approach in a bid to revive rock-bottom poll ratings, Nick Clegg used a round of broadcast interviews earlier to accuse George Osborne of planning to "balance the books on the backs of the poor".
But the Deputy Prime Minister refused to spell out exactly what the Lib Dem "red lines" would be if they find themselves negotiating to enter another coalition with the Conservatives next May.
In a series of barbed assaults on political opponents, Mr Cable told activists that Labour was offering "French Socialism without the sex".
But he reserved his most hostile passages for the Tories, whom he lambasted as "arrogant" and a home for "dodgy billionaires".
Referring to the Chancellor's proposal last month for a further two-year freeze on working age benefits, Mr Cable said: "The need for budget discipline mustn't become an obsession with ever deeper cuts in public spending.
"Key public services have already been cut to the bone from legal aid and local government, to policing and defence. The Tories are ideologically obsessed by cuts because they see it as a way of destroying public service and the welfare state, which they detest.
"The Tories' proposal to take another £25 billion or more out of welfare and unprotected Government departments will do great harm to valuable services: to imagine otherwise is fantasy. I will categorically not go along with this.
"So you might ask what is the alternative? The truth is more taxes will be needed. To contribute to deficit reduction and also to address unacceptable inequalities.
"Any politician who tells you that the next Government can balance the budget and avoid tax increases is lying to you."
Taking a significantly different tone to Mr Osborne on managing the public finances, Mr Cable said that borrowing should be allowed to rise for capital investment.
"There is a role too for more public borrowing by central and local government to finance productive investment in transport, housing and innovation," he said.
"When interest rates are so low, borrowing for investment is a no-brainer and is nothing to do with deficit reduction.
"Of course we need to protect the next generation from too much public (as well as private) debt, but the next generation would certainly not thank us for a legacy of underinvestment, over-stretched infrastructure and unaffordable homes."
And he drew a sharp dividing line over immigration, insisting: "Our responsibility as Liberal Democrats is to tell the uncomfortable truth: that the vast majority of migrants coming to the UK from inside or outside the EU have brought tangible benefits to the economy, importing valuable skills and strengthening our civic culture.
"Of course immigration does have to be controlled at the point of entry and exit. And we must deal with abuse and crack down on 'benefit tourism'. But not at the expense of the EU Single Market, and its free movement of workers."
Pushed on the issue of red lines in possible coalition negotiations next year, Mr Clegg reiterated that he would not allow the Tories to withdraw Britain from the European Convention on Human Rights.
But he appeared to leave wriggle room on Conservative plans for an in-out referendum on EU membership in 2017, stressing that he supported holding such a ballot when a power transfer to Brussels was mooted.
"The Conservative Party are tearing themselves up about Europe so they have now plucked out of thin air an arbitrary date - 2017 - when a referendum would take place," he told the BBC Radio 4 Today programme.
"We have in law a guarantee about when a referendum will take place, when the rules of the game change.
"More than that, I say that the question on the ballot paper should be in or out and I would relish that moment."
Mr Clegg said the Lib Dems wanted to stick roughly to the current coalition approach to deficit reduction in the next parliament - making 80% of savings from spending cuts and 20% from tax increases.
He said the party had already costed some of its proposals on tax and spending in full, such as £1 billion extra NHS funding and scrapping the married couples' tax allowance, unlike the Tories who were "spraying around unfunded moon-on-a-stick" promises.
"What we are doing which is unlike any other party in this conference season - you will search in vain for any costed plans from the Conservatives, they are just spraying around unfunded moon- on-a-stick-type promises," he said.
"We have said there are a number of things we want to do which we have costed in full."
Mr Clegg also said Lib Dem Pensions Minister Steve Webb was "looking at" the possibility of cutting tax relief for pensioners.
Mr Cable said Tory plans to cut spending by £25 billion in the next parliament are a fundamental area of difference with the Lib Dems and said the party would not go along with them.
He told BBC Radio 4's The World At One: "It is a fundamental area of difference but we're not here thinking through the scenario of coalition again, that is an issue which will have to be faced after the election, we respect the views of the public, they are the people who choose the next government, not us.
"We're not talking about red lines or blue lines or yellow lines."
Asked if the Lib Dems would go along with the cuts, Mr Cable said: "We will not (go along with the cuts) but let's separate issues of policy.
"We are absolutely clear, and it's not just about what happens after the election, it's now there are discussions taking place in Government about future spending and the discussions have to take place on sensible assumptions.
"Those figures which were cited at the Conservative conference are not sensible and not things we can go along with."
The Business Secretary also said he was confident the Lib Dems would win seats at next year's general election "in good numbers" as individual MPs are strong in their particular constituencies.
Asked about the party's dismal poll ratings, Mr Cable said: "It is difficult. It's the first time in my lifetime that we have been in government and with government goes difficulties and blame for unpopular policies.
"But I think there is a big difference between the average polls and what we're doing in the constituencies where we are strong and where we have good local MPs and I'm pretty confident that we'll be returned to Parliament in good numbers."
But Mr Cable told Sky News he was not setting any preconditions for future coalition negotiations.
He said: "We're not here talking about red lines in coalition negotiations, that's way ahead.
"There is a general election, the public will decide how many MPs of different parties are elected, what kind of government they want, we respect that.
"After that, there comes the issue of forming governments and coalitions and that's a complex process.
"We're not setting out any preconditions at this stage."
The Business Secretary said he was arguing within Government for more spending in certain areas.
Mr Cable said: "Certainly in the discussions we're having in government, on public spending levels, I'm making it absolutely clear - and I represent an important economic Government department - we have to maintain and in some cases increase the spending we do on supporting innovation and science and training.
"That's the battle I'm fighting in government because I believe we want a strong economy and if that differentiates us from the Chancellor and his party, well so be it."
Mr Cable also condemned the Government's Help to Buy policy, insisting it drove up prices rather than actually helping people on to the housing ladder.
Endorsing a new party policy on boosting housing supply, which includes proposals to allow councils to suspend the right to buy, he said: "How do you stimulate supply? Mr Osborne has this policy called Help to Buy.
"But it doesn't actually help you to buy, because it drives up the price and makes it less affordable.
"What we really need is help to build."
Shadow business secretary Chuka Umunna said: "You can't trust Vince Cable and the Liberal Democrats. They broke their promises and have been too weak to stand up to the Tories.
"They promised to scrap tuition fees and then trebled them, they promised to be on the side of working people, but working people are £1,600 a year worse off since they joined the Tories in government, and they said they wanted fairer taxes but have given millionaires a tax break while everyone else pays more.
"Vince Cable can't run away from his record as part of the Tory-led Government or try to pretend that Tory policies have nothing to do with the Lib Dems. They voted for them and the Tories couldn't have done it without them."
John Longworth, director general of the British Chambers of Commerce, said: "Vince Cable was right to put the economy at the heart of his speech, and issue a warning that we should not become complacent. There is no doubt that tax or cuts, or in fact both, will be needed to deal with the deficit and the growing national debt.
"It is positive to hear the Secretary of State speak of the importance of long-termism in both business investment and in government policy. But too often in the past the rhetoric of politicians is at odds with the reality of what businesses see on the ground. Actions, not words, are what count and this means sticking by an evidence-led approach to policy, not cutting across established mechanisms such as the Low Pay Commission for the minimum wage, and abiding by the findings of the Davies Commission on building new runways."
Katja Hall, CBI deputy director-general, said: "Business agrees that fiscal discipline, industrial strategy, a skilled workforce and an effective migration system are essential building blocks for our economic future. But we must keep our flexible labour market to get more people into work."
On the deficit, she said: "The job is still only half done and tough decisions need to be made. Whoever is in government will need to balance the books while at the same time targeting spending on areas that spur growth, such as capital spending and innovation."
Most voters, 56%, believe the Lib Dems have become irrelevant, according to the ComRes poll for ITV News, though 26% disagree.
Just 14% trust Mr Clegg to keep the promises he makes while 63% believe the Lib Dem leader will break his pledges.
Some 44% of adults believe it was a mistake for the party to go into Coalition with the Conservatives in 2010 and 42% feel betrayed by Nick Clegg.
But ousting the Deputy Prime Minister and replacing him with a new leader would mean just 18% are more likely to vote for the Liberal Democrats.
More voters, however, still give the Liberal Democrats credit for raising of the personal tax allowance to £10,000 at 30% compared to 22% for the Conservatives.
:: ComRes interviewed 2,037 adults online between October 3-5.
Mr Clegg insisted he would not stand down as Lib Dem leader after the next election.
He told BBC 2's Newsnight: "The one thing that I have always kept in mind and I have been diligent about, however many brickbats I might receive, is to keep my party united."
The Lib Dem leader said the rose garden press conference held when the coalition was formed had given a "slightly false impression".
He said: "I think the sense of kind of chumminess, if you like, that may have been given just because it was a sunny day and we gave a sort of positive, relieved press conference as number one and two in the new government, I think that may have sort have slightly given, perhaps, a slightly false impression."
Mr C legg admitted the "ruthless" AV referendum campaign had been a turning point in coalition relations.
He said: "Clearly, the way in which the Conservative party decided to conduct that campaign - in a way which was so unnecessarily personal - they didn't need to do that. They probably have won, frankly, without that because they had all the vested interest in the press up lining up against it anyway. It wasn't getting much of a fair hearing."
He added: "It was, I thought, a totally gratuitous thing. But, in a sense, it has been repeated since then."