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.
Mr Blair and Mr Brown had breakfast with Ikea staff in Croydon
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.
Plans are 'ridiculous'
There are concerns in Labour circles that traditional supporters who were angered by issues like Iraq and tuition fees might vote Lib Dem or for a smaller party in protest, or not vote at all on 5 May.
Mr Blair said while the Lib Dems shared some Labour values, they were "simply incapable" of facing up to hard economic decisions.
"You can, in constituencies like this, vote Lib Dem and let a Tory in. But that's not the only reason I'm saying don't vote Lib Dem," he said.
"I'm saying they don't in truth have a serious programme for government. And that's a reason for not voting for them.
"They have an economic plan that is just ridiculous, thinking you can spend everything you want and the only people who end up paying are a few wealthy people earning over £100,000 a year, it's not serious."
The Conservatives meanwhile had adopted a "spend, spend, spend approach", Mr Brown said.
He said Labour were the only party offering a serious programme for government.
Later he and Mr Blair stopped for breakfast in the staff canteen at Ikea in the marginal Croydon Central seat, before heading to a barbecue at Gillingham, Kent, where Labour supporters were told to "take nothing for granted".
Labour strategists say they have got hold of Conservative campaign documents suggesting the Tories need to win over just 838,000 voters in 165 marginal seats to overturn Labour's majority.
Lib Dem leader Charles Kennedy has denounced Labour's claim that voting for his party would let in a Conservative MP.
"Vote Liberal Democrat and you get Liberal Democrats," he told BBC Radio 4's Today programme.
"What I want people to do is simply to go out there and to register that positive vote for us, safe in the knowledge that we will be seeking to deliver...on the policies we have put forward."
Conservative leader Michael Howard told party activists on Monday he was confident his party could win.
"I believe our message is getting across. People know what is needed for the future of our country and they have a very clear choice on Thursday," he said.
The Tories also say Labour plans would leave a "gaping black hole" in public finances.