The strained relations between the Conservatives and Liberal Democrats reached a new low yesterday amid an increasingly bitter campaign ahead of next month's referendum on the voting system.  Doubts were expressed about whether the Coalition would last as planned until 2015 after Nick Clegg launched his strongest attack on the Tory-led No campaign and did not exempt David Cameron from criticism. Officially, both parties insisted it will be "business as usual" inside the Government after the 5 May referendum. But insiders believe the scars from the referendum battle will mean that the relationship between the Coalition partners will never be the same again.  The Liberal Democrats raised the stakes yesterday as they sought to combat what Mr Clegg called the "lies, misinformation and deceit" of the No camp. He told The Independent on Sunday that opponents of the alternative vote (aV), including Mr Cameron, were the "death rattle of a right-wing ?lite, a right-wing clique who want to keep things the way they are".  Simon Hughes, the deputy leader of the Liberal Democrats, will complain to the elections watchdog the Electoral Commission about "untruths" pumped out by the No campaign.  Chris Huhne, the Liberal Democrat Energy and Climate Change Secretary, suggested his party might sue the No to aV organisation unless it retracted its claims about aV - including a statement that it would mean spending ?135m on electronic vote-counting machines. He warned that the credibility of Mr Cameron, the Chancellor, George Osborne, and the Foreign Secretary, William Hague, would be undermined unless the allegations were withdrawn.  He told the BBC: "Let's be clear - if they aren't prepared to come out with any substantiation for this extraordinary allegation that we're going to need voting machines for the system when none of the other countries actually have [them], there is a very simple legal redress. My colleague Simon Hughes today is talking about getting the Electoral Commission to look at this, and there will be other legal means as well - so they'd better come clean pretty fast."  Mr Huhne refused to say whether he might resign from the Cabinet over the aV row. "The key point is that we have a very difficult job to do, and obviously it is substantially easier to do if we have a good personal relationship - and it is, frankly, worrying if you have colleagues who you've respected and who you've worked well with who are making claims which have no foundation in truth whatsoever. If they don't come clean on this, I'm sure the law courts will."  Senior Tories and Liberal Democrats expected the two parties to diverge ahead of 5 May, when they will do battle in elections to English councils, the Scottish Parliament and Welsh assembly as well as on opposite sides of the aV campaign. But the acrimony of the battle over voting reform has surprised both sides. "We have both gone off script," one source said. "We didn't think it would get this nasty. It's a learning curve for both of us."  In some ways, it suited both parties to have a degree of what marketing men call "product differentiation" ahead of 5 May. If anything, the Coalition had worked so well that there were fears in both parties that Mr Cameron and Mr Clegg were a bit too close for their parties' own good.  a lot has happened since then. The Li