Targeting Liberal Democrat voters is the most effective way of attacking the coalition government, David Miliband has told politics.co.uk.
The Labour frontrunner, looking ahead to the party's task in opposition after its next leader is unveiled on September 25th, said the immediate priority is to "expose what the Tories are doing wrong" - both to voters and Lib Dems.
"You need 56 Liberal Democrat MPs to pass any piece of legislation. A lot of what they're voting for they're actually campaigning against," he said in an interview with politics.co.uk published today.
Pressed on what steps he would take to undermine the coalition by attacking the Lib Dems, he added: "I'd be targeting their voters. Nothing concentrates the mind like the sinking feeling that your voters are disappearing."
His comments come ahead of what could be a fractious week in Liverpool for the junior coalition partners.
Nick Clegg and the party's ministers are preparing to confront left-leaning grassroots activists who face a tough fight against Labour in next May's local elections.
Mr Miliband believes Lib Dem tensions hold the key to bringing down the government, but his comments may come as a surprise to the party's left-wing figurehead Jon Cruddas, who endorsed David Miliband over Ed Miliband last month.
"Attacking the Liberals is wrong. There's a danger of us spraying too much lead across the forecourt and not really thinking about how we need to regroup," Mr Cruddas said.
Mr Miliband argued Labour can only make progress by improving the party's organisation outside London.
"We can win arguments in Westminster but unless we are mobilised and organised in constituencies in Westminster we won't make progress," he said.
The shadow foreign secretary is committed to reforming Labour's organisation outside London, insisting opposition to the coalition has "got to be driven from the outside of Westminster in as well as from the inside of Westminster out".
He said the party would benefit from the third of his leadership campaign funds which will be transferred to a fighting fund for Labour, pointing out "it's an important job of a Labour leader to raise money".
A poll published this weekend suggested the momentum may be shifting towards Mr Miliband's brother, although the reliance on uncertain second preference votes clouded its reliability.
"It's been a good contest," Mr Miliband commented.
"It's quite interesting to have an election where the polls are relatively rare, where a lot of the electorate are hard to find. There's all to play for."