Nick Clegg has battled party activists over cuts
The Lib Dems have backed leader Nick Clegg's call to cut any election pledges found to be too expensive, as they launched their draft manifesto.
Mr Clegg says he wants to be straight with voters about the savings needed to balance Britain's books.
But his recent focus on the need for cuts was criticised by several MPs.
Pensions spokesman Steve Webb warned Mr Clegg that the party must offer a more "positive" message and stop preaching "doom and gloom".
The Fresh Start for Britain document, which was endorsed by the party's conference in Bournemouth, includes a detailed list of policy ideas on everything from scrapping Trident to affordable housing and replacing road tax with road pricing schemes.
It also includes a pledge to scrap university tuition fees - but warns that this and other policies may be downgraded to long-term aspirations or dropped from the final manifesto altogether.
It says: "Those policies which require public investment will only be introduced as and when resources can be identified by cutting public spending elsewhere.
"We will publish detailed costings to demonstrate how to pay for all our policy priorities without increasing public spending at the time of the general election.
"The manifesto will be a fully costed programme for government, so any policy for which resources cannot be identified will be excluded."
'Doom and gloom'
By approving A Fresh Start for Britain, Mr Clegg said the Lib Dems had asserted the values which would inform the tough decisions the next government would have to take.
"Political parties have to make difficult choices - where to cut, where to spend what little money there is," he said.
"There is still hope for a different, better future - if we are brave enough to make a fresh start."
But Mr Webb said persistent talk of cuts could "alienate" voters at the election and warned against introducing means testing on benefits, something the leadership has floated as an idea.
"I think we have overdone the despair," Mr Webb said. "Doom and gloom does not inspire and motivate people. People want it straight but they also want hope."
Mr Clegg's comments that "savage" cuts in some budgets may be needed to protect funding in priority areas has caused unease in sections of the party - even though he has stressed that there would not be an overall reduction in public spending under the Lib Dems.
The man responsible for putting together their manifesto told members that the party would not make any election promises which were not affordable.
Lib Dem policies must be "economically credible" as well as "politically bold and socially radical", Danny Alexander - Mr Clegg's chief of staff - said.
Mr Alexander said the party was setting out its priorities for building a sustainable economic recovery, making society fairer and cleaning up politics at a time when the nation's finances had been badly damaged by the financial crisis and recession.
Firmly held principles
He warned members that the UK's debt crisis and the need to cut spending meant that not all "long-cherished" principles could be delivered.
"We must not make any firm commitment to the British people for the next Parliament unless we are certain it can be paid for," he said.
Mr Alexander stressed no final decisions had been taken on key issues like public sector pay and pensions but said the party must be mindful of whether "firmly held" principles such as scrapping tuition fees and free care for the elderly were affordable in the immediate future.
'FRESH START' POLICIES
Reduce class sizes to a maximum of 15 pupils
Cut road tax by 50% for drivers in rural areas
Pay junior soldiers the same as police constables and fire fighters
The debate comes amid opposition among grassroots supporters - and senior figures such as Charles Kennedy - to suggestions the tuition fees pledge could be dropped and Mr Clegg's "savage cuts" comments.
Martin Horwood, MP for Cheltenham, said the party should make a "stronger and clearer" commitment to scrapping tuition fees rather than putting it on hold.
Evan Harris, of the Federal Policy Committee, said his group would still have the final say over what went in the manifesto and he expected the tuition fees pledge to remain intact when it was considered in the run-up to the election.
He told members that while Mr Clegg was a "really good" leader, "great" Lib Dem leaders recognised that decisions about policy-making ultimately rested with the party.
There has been growing unrest over the party's new policy to tax homes worth more than £1m, with senior MPs expressing anger over its practicality and how it was announced on Monday.
While the party recognised the need for some spending cuts, Mr Harris said: "We do that with a heavy heart, with care and caution, with no enthusiasm and as a last resort."
Ahead of Mr Clegg's keynote speech to conference on Wednesday, a poll for BBC Newsnight suggests a third of voters have never heard of him.
When asked what they thought of Mr Clegg, 36% said they had a favourable opinion, 28% said the opposite and 36% said they did not know who he was.
Mr Clegg also received lukewarm support among those who voted Lib Dem at the 2005 general election. Just one in three believed he would make the best prime minister out of the main party leaders - compared to two-fifths who backed Tory leader David Cameron.
Orb interviewed 1,056 adults from across the UK between September 11 and 14.