David Cameron warned Britain faced political and economic "stagnation" if the General Election returned a hung parliament, as Nick Clegg claimed the era of the old two-party "stitch-up" was over for good.
With the Liberal Democrats breathing down the necks of the Tories in the opinion polls, Mr Cameron said only an outright Conservative victory on May 6 could deliver a strong government capable of taking the necessary decisions on the economy.
In his starkest warning yet of the dangers of voting Lib Dem, he said the rest of the world would not understand how Britain could tackle its record deficit unless the election produced a decisive result.
"Any other outcome, any other vote, could lead to a sort of stagnation, to a sort of haggling and a bickering amongst politicians and we won't get done what so badly needs to be done in our country," he said during a campaign visit to a brewery in Burton-on-Trent. "If we end up with a hung parliament, the world is going to look at us and say 'How are you going to sort out your problems?' We need decisive action to sort out those problems and that is what a Conservative government would bring."
His warning reflected the deepening concern within the Tory high command over how to combat the Lib Dem resurgence which is threatening to derail their hopes of a return to government after 13 years in the political wilderness.
Recent polls have put the Lib Dems in second place, with backing between 28% and 31%, just behind the Conservatives scoring 32%-33%, with Labour on 26%-28%.
Speaking at the Foreign Press Association in London, Mr Clegg said there had been an "irreversible" shift in the political landscape. He said: "We are never going to turn the clock back as Gordon Brown and David Cameron want to do, to that cosy little stitch-up between the two parties where they think only they can speak on behalf of the nation. That's gone, that world is gone."
During a visit to the University of Worcester, Chancellor Alistair Darling echoed the Tory line that a vote for the Lib Dems represented an "unacceptable risk" at a time of economic uncertainty.
"We have come through a very difficult time over the last couple of years and we are now coming out of it. To throw that away and take an unacceptable risk is just not worth taking. The country is going to have to make some pretty big decisions about where we go over the next 10 to 20 years and I believe it is absolutely imperative that we have a strong government that knows where it is going and is prepared to do what is necessary to secure jobs in the future."
However Business Secretary Lord Mandelson said that Mr Clegg should be taken "seriously", and appeared to reach out to Lib Dems by underlining Labour's commitment to reform of the electoral system.