Nick Clegg says he will push to ensure no one on the minimum wage pays income tax if the Lib Dems are in government after the next election.
The Lib Dem leader said coalition government was good for the UK but did not indicate whether he favoured partnership with the Tories or Labour.
He refused to spell out his red lines - the policies he would "die in a trench for" in coalition negotiations.
But he said tax fairness would continue to be the party's "signature tune".
Mr Clegg told the BBC's Andrew Marr programme coalition governments would be more likely in future rather than "these slam-dunk results where one or the other of the two major parties always get a majority"
Speaking from the Lib Dem annual conference in Glasgow, he said political parties needed to be "up-front with the British people about those issues which we really will die in the trench for and those which clearly will depend on political and economic circumstance".
He declined to say, more than 18 months ahead of the election, which policies would be his "red lines" in any coalition talks.
'Finish the job'
But he said: "I can give you a clue that... tax fairness will of course be one of the signature tunes for the Liberal Democrats."
"We are committed as a party - and I am committed to this - to raising the allowance further such that... everybody on the minimum wage pays no income tax."
The personal allowance rose to £9,205 in April and to £10,000 in 2014, fulfilling a key Lib Dem demand in negotiations over the coalition agreement. Ensuring all those on the minimum wage paid no income tax would mean increasing the threshold to £11,400.
Mr Clegg said the UK needed Lib Dems in government because they would act as a moderating influence on the bigger parties, telling Andrew Marr: "If we go back to the bad old days, not of coalition or balanced politics, but of either the left or the right dominating government on their own, you will get a recovery which is neither fair nor sustainable.
"I think Labour would wreck the recovery, and under the Conservatives - who don't have the same commitment to fairness which we do - you would get the wrong kind of recovery.
"Our message is that coalition is good, for the Liberal Democrats to stay in government is good, let us finish the job but let us finish it fairly."
He said Labour needed to "spell out" what they believed in.
But he added that he was "absolutely not" already discussing a second coalition deal with the Tories, because, "you have to let the British people have their say first."
He said his party's commitment to a "mansion tax" on properties over £2m, raising £2bn, sent a signal that "even though we are committed to deficit reduction" that would not be achieved entirely through spending cuts.
Mr Clegg also rejected fears - expressed by Business Secretary Vince Cable - a government scheme to back mortgages would lead to another unsustainable house price bubble but admitted ministers would have to be "vigilant" to ensure that did not happen.
He also attacked Home Secretary Theresa May's immigration policies, describing an advertising van urging illegal immigrants to "go home" as "silly" and ineffective.
He said talks were still going on behind the scenes about plans for a cash bond to be paid by some overseas visitors to ensure they returned home when their visas expired.
Mr Clegg wants the bond to be set at £1,000 and be offered to visitors from "high-risk" countries who have been refused a visa as a "discretionary tool" for immigration officers.
Ms May wants a £3,000 bond for all visitors from so-called "high-risk" countries.
Also at the party conference on Sunday, Liberal Democrats voted to support the building of a new generation of nuclear power plants - a policy U-turn which marks an important victory for the party's leadership.
Climate Change Secretary Ed Davey had urged activists to drop their traditional opposition to nuclear energy.
Delegates also approved changes to party rules on bullying, harassment and intimidation.
It comes in the wake of a report into allegations of sexual impropriety by the Lib Dems' former chief executive Lord Rennard. He has denied the allegations.
The new rules state party members must not "bully, harass or intimidate", but Tessa Munt, parliamentary aide to Mr Cable, said reforms did not go far enough and suggested party members should have a "duty to take action" if they witnessed harassment.
Labour dismissed Mr Clegg's talk of more new homes saying the UK had seen the biggest fall in housebuilding in 30 years under the coalition
Deputy leader Harriet Harman said: "Nick Clegg is desperately trying to distance himself from the failures of David Cameron's government.
"The truth is that he has broken his promises, and backed the Tories all the way. Nick Clegg and his party must take responsibility for their government's failure which has caused the cost of living crisis families are facing."