Labour leader Ed Miliband appealed to voters today not to treat tomorrow's poll on electoral reform as a referendum on Nick Clegg.
Campaigners were making a last pitch for support in the battle over whether to adopt the Alternative Vote (AV) system for electing MPs, as well as the fight for English councils and devolved assemblies in Scotland, Northern Ireland and Wales.
Mr Miliband, who supports the move to AV - under which voters would rank candidates in order of preference - said it was a "once-in-a-generation opportunity" to change Britain's political system for the better.
The Labour leader appealed to voters to use tomorrow's elections to send a message to the coalition Government about their concerns over cuts, university tuition fees and the proposed shake-up of the NHS.
But he urged them not to seek to punish the Deputy Prime Minister for breaking Liberal Democrat election promises by voting down AV in the referendum, which was the key concession he gained from David Cameron in last year's coalition negotiations.
Mr Miliband told BBC Radio 5 Live: "The reason I did not share a platform with Nick Clegg is because I didn't want it to become what in some people's minds I think it still is, which is a referendum on Nick Clegg and some of the promises he has broken.
"This isn't about Nick Clegg. This is about whether we change our politics for the better and this is a once-in-a-generation opportunity to do it."
A ComRes poll for The Independent today showed a commanding 32-point lead for the No campaign among those who have made up their minds and say they are sure to vote in the AV referendum.
It put the No vote on 66% compared with 34% for the Yes campaign - a 12-point widening of the gap since last week, when the margin was 60% to 40%.
Mr Miliband told Radio 5 Live why he backed a Yes vote: "This is a chance to change our political system for the better, in my view, a chance to ensure that every MP elected must seek 50% of the vote, a chance to make more people's views count and a chance to change the yah-boo political culture, because we will all have to reach out more."
The Labour leader said a good result in Thursday's elections will not guarantee that it is on the route back to power, and acknowledged that his party has yet to regain the trust of voters.
"We are winning it back by talking about the issues that people care about," he said, citing squeezed living standards, prospects for young people and building stronger communities.
"It's by talking about these issues that we start to regain trust, but it is a process and I don't say it will be completed after these local elections, whatever the outcome.
"The basic point that we shouldn't assume that any gains we make in local elections mean we are on course to win the next general election is right. You will find me over the coming years as the eternal warrior against complacency, because I know my party has a long way to go to regain trust with people."
Mr Miliband added: "These elections are an opportunity for people to send a message about the direction this Government is going. They are doing things people didn't vote for.
"This isn't a majority Tory Government and if you look at what they are doing on the NHS, on tuition fees, on the deficit, a majority of people didn't vote for these changes at the last election.
"I hope people will use the opportunity of these local elections to elect good local Labour councillors but also to send a message to this Government."
Mr Clegg conceded that feelings were "rising high" in Westminster but sought to divert attention towards the issues - telling voters AV was "a bit better" than first past the post.
Tensions between the governing coalition partners erupted across the Cabinet table yesterday when Energy Secretary Chris Huhne directly confronted Prime Minister David Cameron and Chancellor George Osborne over alleged "lies" by the No camp.
The Deputy Prime Minister declined to comment directly on his Lib Dem colleague's intervention - the latest in a succession of vociferous criticisms - which was criticised by the Yes campaign yesterday as unhelpful.
But he told BBC Radio 5 Live: "The temperature is rising, feelings are rising high as you would expect at this stage of a referendum campaign but at the end of the day it isn't about what politicians think or feel or even say to each other, it's about what people want.
"If you basically think the current system is absolutely fine, totally perfect, nothing wrong with it then obviously vote No and stick with what we've got.
"If you want something a bit fairer, a bit better, which makes all politicians work a bit harder for your vote then vote Yes, vote for change."
Many people would not put their minds to how to vote until tomorrow, he suggested, refusing to concede that defeat appeared likely.
"Gallons of ink is going to be spilled in the nation's newspapers providing a post-mortem one way or another. Actually the choice before people is not whether this was said in the referendum campaign or that was said in the referendum campaign but what this is all about."
The coalition would "move on" whichever way the referendum went and continue dealing with priorities such as addressing the UK's record deficit, he insisted.
Mr Miliband acknowledged that there were party political calculations, as well as arguments of principle, behind his backing for AV.
And he admitted that Labour did not introduce it during its time in power because it had a large enough majority not to need it.
The Labour leader insisted that he did not see the vote on AV as a stepping stone to a proportional representation (PR) system, which he opposes.
And he made clear that a No vote tomorrow will put electoral reform off the agenda for a long time.
"I don't think we are going to be coming back to this very quickly," he told BBC Radio 4's Today programme.
Asked if a No outcome would mean "that's it" for electoral reform, he replied: "Well, yeah, but let's see what the result is."
In a frank discussion of the electoral impact of AV on the different parties, Mr Miliband said: "I do think there is an anti-Conservative majority in this country and I don't think first-past-the-post gives expression to it.
"My main reasons for advocating a change are principled reasons to do with a fairer system. I also happen to believe there is a progressive majority in this country and first-past-the-post doesn't give expression to it."
Challenged over why Labour did not introduce AV during its 13 years in power, Mr Miliband said: "We should have done. It is something I pushed for in relation to the manifesto, which I was responsible for, and indeed it was in the manifesto."
But he admitted: "We had too big a majority. When a Government comes along and has a majority of 170, there is not much incentive to change the electoral system."
During a visit to Leicester, where voters are electing a mayor and a new MP for Leicester South as well as taking part in council polls and the AV referendum, Mr Clegg was asked what he would do if Lib Dem losses damaged the credibility of the coalition.
He replied: "You know, what I've never done - and I'm not going to do it now - is to start playing this 'what if' game the day before the election.
"Tomorrow is an important day for people to ask themselves who is the best placed in a local community to work for local families, to work for local communities.
"As I travel around the country I am so impressed by the hard work, the dedication, the commitment of thousands of Liberal Democrat campaigners whose only care in the world is what is best for their own communities and I hope that will be recognised by people when they vote tomorrow."
Asked whether Mr Huhne's comments in Cabinet yesterday suggested he was positioning himself to quit the Government and become a leadership rival from the backbenches, Mr Clegg said: "I think temperatures of course run high in the final stages of a referendum campaign.
"At the end of the day, this isn't about what one politician said to another, it's not even about what one political party thinks about it, it's what do people think when they have the chance to cast their vote at the referendum tomorrow.
"I'm not going to provide a running commentary on Cabinet. Clearly temperatures run high at this stage in the referendum campaign but the great thing about a referendum is it's not about the politicians, it's about what the people want."
Mr Cameron defended the existing voting system in the Commons against charges by one Lib Dem MP that it produced results "that would embarrass Robert Mugabe".
Colchester MP Bob Russell said that at last year's general election, Tories polled 49% of the votes in Essex but won 95% of the seats.
"Other than that Essex is now a Labour-free zone, do you think that result was fair?" he asked.
The Prime Minister told him: "In Colchester, everyone had one vote, it was counted once, and you won. I congratulate you.
"In other parts of Essex everyone had one vote, they were all counted once and many of my honourable friends won."
He also insisted that, despite the differences over AV, there was "as good an argument" today for the coalition as when it was formed.
"Of course we do not agree about the future of our electoral system. We are having a referendum, we are having a debate about it," he told MPs.
"But the reason for having a coalition Government, coming together, sorting out this country's problems in the national interest, is as good an argument today as it was a year ago when we went into government to clear up the mess that was made by the party opposite."
Former foreign secretary David Miliband blamed "personality infighting" inside the coalition for the failure of the pro-AV camp to enthuse voters.
"The personality infighting in the coalition has rather got in the way of the real issues," he told the Boulton & Co show on Sky News.
"I was at Sussex University yesterday with 400 students and they were complaining that there was not proper engagement with the issues - and, by the way, they were nine to one in favour of change, which was surprising to me as I can read the polls as well as you can."
Mr Miliband joked that he might be seen to have "more reasons than some to be dubious about AV" about his shock defeat by brother Ed in the Labour leadership election using the system.
But he went on: "We have an opportunity tomorrow to change our voting system in a way that enfranchises more people in Britain and ensures that every MP gets 50% of the vote."