Liberal Democrat leader Nick Clegg dismissed Labour promises to overhaul the voting system, likening them to a "consumer guarantee from Del Boy".
Gordon Brown is expected to use a keynote campaign speech to highlight Labour's plans for constitutional reform - including a referendum on introducing a form of proportional representation for Westminster.
The proposal has widely been seen as an attempt to woo the Liberal Democrats in the event of a hung Parliament - although Labour's plans do not go as far as the Lib Dems would like.
However Mr Clegg insisted that neither Labour nor the Conservatives could be trusted when it came to issues of constitutional reform.
"They have systematically at every turn blocked every single reform - they have blocked party funding reform, they have blocked reform on lobbying," he told the BBC Radio 4 Today programme.
"Believing any promises from them on political reform is a bit like accepting a consumer service guarantee from Del Boy - don't believe it, they are trying to treat you like fools."
Mr Clegg also said he believed there was a long-term case for Britain to consider joining the euro, although he acknowledged that in recent years interest rates in the eurozone would not have been right for the UK.
"I don't think the euro is for now. I would go even further and say that interest rates under the eurozone wouldn't have been right for the British economy," he said. "We think there is a long-term case for considering entry into the euro, but of course that needs to be done on the back of a referendum."
He also defended the Lib Dems' plans to scrap the update of the Trident nuclear deterrent, saying it was unnecessary and unaffordable. He insisted however that he was not arguing for unilateral nuclear disarmament, and said his party was looking at cheaper options to replace Trident.
"Whether we have a nuclear deterrent or not depends heavily on whether multilateral nuclear disarmament in the non-proliferation talks succeeds or not," he said. "I am a multilateralist, I believe that we should be not just throwing away our nuclear deterrent without any regard to what others are doing."