Vince Cable today makes a final plea for people to change the voting system to prevent the "forces of reaction" in the Conservative Party and the right-wing press dominating the 21st century as they did the 20th.
In a provocative article for The Independent on Sunday, the Business Secretary says the country has a "historic opportunity" to back the alternative vote system in the referendum this Thursday before the chance of reform is lost for ever.
Directly challenging the Prime Minister and further testing the stability of the coalition after weeks of infighting, the Cabinet minister says replacing first past the post with AV would "prevent repeated unrepresentative government of the political right".
Voting Yes to AV would ensure the next 100 years was not another "grey Conservative-dominated century" like the last, Mr Cable says.
Mr Cable's party colleague, Chris Huhne joins the onslaught with a public call for voters to form an anti-Tory alliance, in order to deprive the Conservatives of power.
In a joint article with Labour shadow minister John Denham, and Green Party leader Caroline Lucas – published in The Observer – the Energy Secretary said the referendum was an opportunity for the country's "progressive majority" to back change and avoid a repeat of the "worst excesses of the Thatcher government".
With five days to go in the bitter referendum campaign, and with the No camp 20 points ahead in the polls, there is unease across both Lib Dem and Tory parties this weekend at what the expected result will mean for the future of the coalition, with Nick Clegg's ability to shape government policy in question.
A new survey in the Mail on Sunday today showed 51 per cent of voters polled said they were opposed to the reform, while only a third supported it. The Bpix research also found that two out of three voters believed defeat on Thursday would cause lasting damage to Mr Clegg and his party.
Meanwhile, a dossier by the Yes campaign highlighted fears that the current system allows MPs with comfortable majorities to regard their jobs as being for life. Figures showed a clear link between safer seats of all parties and higher outside earnings of MPs. Average earnings on top of salary for an MP in a safe seat were £11,000 last year, compared with £6,500 for an MP in a marginal seat.
Martin Bell, the former independent MP for Tatton and anti-sleaze campaigner, said the dossier by the campaign group Yes to Fairer Votes exposed the existing system as a "moonlighter's charter". The Yes campaign claims AV will make MPs work harder for their constituents in order to reach out to 50 per cent of voters.
Mr Bell said: "An MP in a safe seat, enjoying its benefits, can leave constituency cares behind and seek employment elsewhere. An MP who faces the threat of unemployment at the next election will be less tempted to graze in the pastures of lucrative consultancy. MPs' time-sharing short-changes their constituents."
In his article, Mr Cable says: "The forces of reaction have been impressively marshalled on the battlefield. Not a single Conservative parliamentarian has broken ranks in an uncompromising defence of the status quo.
"The country's right-wing newspapers – both the Murdoch and non-Murdoch titles – have swallowed their dislike of the coalition's liberal compromises, and of each other, to line up solidly behind the No campaign.
"There is a real danger that because of factional dislikes on the progressive side of British politics, we shall again miss a historic opportunity."
Appealing to voters to look at the "big picture", Mr Cable adds that the 20th century produced two "great reforming governments" – the 1906 Liberal administration which created the old age pension, and the 1945 Attlee government, which launched the NHS.
"These governments were brief interludes of sunlight in a grey Conservative-dominated century," says Mr Cable.
"Labour, Lib Dem and Green supporters should ask themselves one simple question before they vote: why is it that the Conservatives are pulling out all the stops, with their millionaire backers pouring the contents of their coffers into the No campaign?
"It is because they know that first past the post is stacked in their favour and they are determined to keep it."
MPs on the Tory right are fearful Mr Cameron will be tempted to offer Mr Clegg more Lib Dem-friendly policies as a consolation prize for losing. The Lib Dem leader has borne the brunt of personal attacks from the No campaign, and the Prime Minister will be tempted to soften the blow. Right-wingers have warned Mr Cameron that after a year of government U-turns and the decision to stall NHS reform plans, he must use a No result to strengthen his hand.
However, a Lib Dem aide made clear last night that Mr Clegg would seek to extract more from the Prime Minister. The Government's controversial NHS reforms are likely to be at the heart of negotiations.
While Mr Clegg is unlikely to face a serious challenge from within his party to his leadership, he will be left badly wounded by a No result.
The Labour leader, Ed Miliband, whose party remains split over the referendum, was already looking beyond Thursday's vote.
Campaigning for the local elections in Ashfield, Nottinghamshire, tomorrow, Mr Miliband will say the coalition is a "government without a mandate", as reorganisation of the NHS and cuts to frontline public services were in neither Tory nor Lib Dem manifestos. Appealing to Lib Dem voters to back Labour in this week's elections, the Labour leader will declare that his party is not the one Gordon Brown led to defeat a year ago.
Mr Miliband will say: "Over the past year it has become clear this is a Conservative-led Government operating with a substantial Commons majority but – crucially – no mandate for its programme on the economy and the NHS.
"But the Lib Dem strategy of private obedience and public shows of dissent is neither dignified nor effective.
"This week, people are being given a chance to deliver a verdict on a year of a Conservative-led Government and the willing participation of the Liberal Democrats within it. Labour has changed as a party ... we are a party people are coming towards, not turning away from."
But the Labour leadership was already looking beyond 5 May in anticipation of a No vote.
A poll by Survation of 526 constituents in Leicester South, where a by-election is also taking place this Thursday, put the No vote on 56 per cent, 12 points ahead of the Yes vote. It also suggested that Labour would comfortably retain the seat.
In a sign of how Labour is split over the referendum, Blairite former ministers Gisela Stuart and Lord Rooker, supporters of full proportional representation, condemned AV as an "even more flawed, disproportional system" than FPTP. They said it was a "tragedy" that full PR was not being offered to the electorate on 5 May.
The Yes lobby will attempt one final push. A rally in London on Tuesday evening will see Lord Ashdown, Alan Johnson and Eddie Izzard make a last-ditch appeal for votes.
Britain's voting systems: All Thursday's polls – how they work
Alternative Vote referendum The chance of electing MPs by ranking candidates in order – with second (then third, etc) preferences shared out until one gathers half the vote – was a concession in the coalition agreement. It will be decided by a simple run-off of the Yes and No options – with the Nos currently enjoying a big lead.
Scottish Parliament The SNP expects to profit from its rivals' problems to tighten its grip on power; 129 MSPs are elected under the hybrid Additional Member System (AMS). Voters elect a constituency member by first past the post, and seven more MSPs for their region from a closed list.
Northern Ireland Assembly The Sinn Fein-DUP accommodation should continue after Single Transferable Vote (STV) – ranking candidates by preference – produces six members (MLAs) for each of 18 constituencies.
Northern Ireland districts Voters will also elect 26 councils by STV.
National Assembly for Wales Labour hopes to retain power, probably in coalition. The AMS produces 40 constituency Assembly Members, plus five more from each of four regions.
English councils The Tories and Lib Dems defend huge gains from 2007, when 279 councils are elected by first past the post. Labour must win at least 1,000 seats to claim progress.
Leicester South by-election Labour has a huge lead in the race to elect a new MP by first past the post.
Mayoral elections Bedford, Mansfield, Middlesbrough, Torbay and Leicester will elect mayors by Supplementary Vote (this is like AV, but with voters allowed only two preference rankings).