Mr Clegg accused opponents of using "tired" arguments
Labour and the Tories have targeted Lib Dem policies as opinion polls suggest Nick Clegg is still enjoying a rise in support after the TV debate.
Tory leader David Cameron said plans for a selective amnesty for illegal immigrants were a "huge mistake".
Labour's Gordon Brown said voters would "think twice" about Lib Dem plans for child tax credits and Trident.
But Mr Clegg dismissed suggestions a Lib Dem vote would result in a Labour government as a "tired old claim".
In other political news on Monday:
A YouGov poll for the Sun, carried out on Saturday and Sunday, suggests the Lib Dems are one point ahead of the Tories on 33% with Labour in third place on 26%.
Another poll, carried out by ICM for the Guardian, suggests Mr Clegg's party has gained 10 points since last week, giving it 30% - three points behind the Tories but two ahead of Labour.
A ComRes survey for ITV News and the Independent, released on Monday evening, suggests Labour and the Lib Dems are both on 28%, with the Conservatives leading on 32%. A total of 1,003 adults were interviewed by phone for the poll on Saturday and Sunday.
Professor John Curtice, of Strathclyde University, told BBC Radio 4's World at One programme that of the previous five opinion polls conducted since the prime ministerial TV debate, on average they put the Conservatives on 32%, the Lib Dems on 31% and Labour on 28%.
But because the Lib Dem vote was more "geographically evenly spread", under the first-past-the-post voting system, that percentage of the vote would not translate into a third of seats for the party - but pointed to a hung parliament.
In response, the Conservatives have dropped a planned political broadcast attacking Labour in favour of a personal plea from Mr Cameron to voters to avoid such a scenario.
The Tory leader said on Monday that would lead to "uncertainty, fudge and division" and could mean people end up being "stuck with Gordon Brown in Downing Street".
He acknowledged that after the TV debate the election "started to get a bit lively", and people wanted "something different" but said only an outright Conservative victory would bring "real change".
Mr Cameron said a Lib Dem policy giving illegal immigrants the right to earn British citizenship was a "huge mistake" and evidence from other countries suggested it would encourage more illegal immigration.
But he said he would not turn to negative campaigning and would instead "redouble the positive".
However former party chairman Lord Tebbit told BBC Radio 4's The World At One: "There is a Clegg bubble - the important thing is to puncture the bubble before 6th May - the 7th would be too late."
Shadow defence secretary Liam Fox told the programme they need to "ask questions" of Lib Dem policies on issues like immigration, crime and the euro.
The spirit of Dunkirk has gripped the nation. The country has found a politician it likes even more than Winston Churchill, according to one poll
"The Liberals are very good at trying to be all things to all people and when you start to scratch below the surface, there are a lot of gaps," he said.
At a Labour press conference Gordon Brown suggested the Lib Dems' new popularity was a "political honeymoon".
But he said while there was "common ground" between Labour and the Lib Dems on issues like political reform - there were differences on economic policy. He said when people looked more closely at their policies they would "think twice".
He said: "We believe we have a manifesto for a Parliament, we believe we are the people to take this country through the recession and into recovery... We are fighting for victory and for a majority at this election."
Mr Brown said the TV debate had "energised" the election campaign but predicted that the focus would switch from "style and presentation" to "substance".
'Tired old claim'
Mr Clegg said both Labour and the Conservatives were obviously going to "lash out" and both had used the "tired old claim" that a vote for the Lib Dems would benefit the other party.
"A vote for the Liberal Democrats is exactly what it says on the tin - it is a vote for the Liberal Democrats," he said.
He defended his immigration policy, denying that he advocated an "unconditional" amnesty for illegal immigrants.
Rather, the Lib Dems' plan for earned citizenship for illegal immigrants who have been resident for 10 years was "smart, tough and realistic", he added.
On Trident, Mr Clegg said he had discussed the policy with senior defence figures including former chief of the defence staff Lord Guthrie of Craigiebank.
The Lib Dem leader said there were potentially cheaper options than spending £100bn on a like-for-like replacement for the submarine-based nuclear missile system and that the decision need not be taken immediately.
On his party's election prospects, Mr Clegg said there was a "fluidity in this election which we haven't seen for perhaps a generation" and at only two other elections in the last 30 years had there been "the sense that people might move in a big way" - in 1983 and 1997.