Tony Blair and Gordon Brown are trying to drum up support in marginal seats amid fears disillusioned Labour voters will not turn out to back them.
They say if one in 10 Labour voters stays at home or votes Lib Dem, the result could be a Tory government.
Mr Blair said the Lib Dems' plans were "not credible" and only Labour could be trusted with the economy.
The Lib Dems say the votes claim is "rubbish". The Tories say there is a black hole in Labour's finance plans.
The prime minister attacked Lib Dem and Tory economic plans on a visit to Wimbledon, south-west London, held by Labour MP Roger Casale with a majority
of 3,744 in 2001.
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