Chancellor George Osborne has accused Ed Miliband of "an assault on everyday working people", while the Labour leader said Tories were only interested in "looking after a few big firms and individuals at the top".
As they exhanged blows in the run-up to the general election, Mr Miliband launched a flagship Labour policy which would outlaw most zero-hours contracts by giving employees the right to a regular contract after 12 weeks of working regular hours.
But he was forced onto the defensive by a letter from 103 business leaders to the Daily Telegraph, which praised the "Conservative-led government" for supporting investment and job creation and lowering the main rate of corporation tax to 20p.
Chancellor George Osborne said the "unprecedented" letter showed that Conservatives were offering "business stability", while Labour's pledge to return the main rate of corporation tax to 21p represented "a huge risk to the British economy and to British jobs".
The letter was particularly awkward for Labour, as its signatories included at least five previous party backers, including Dragons' Den star Duncan Bannatyne and theatrical impresario Sir Cameron Mackintosh.
But Labour pointed out the signatories had between them donated £9m to the Tories, while several had received honours, including Conservative peers Lord (Stuart) Rose, Lord Bamford and Baroness (Karren) Brady. Shadow business secretary Chuka Umunna said they represented "0.002%" of the country's five million companies, most of which would gain from Labour's plans to cut and then freeze business rates.