Ed Balls sent Business Secretary Vince Cable a political love letter at the weekend, but left-wing activists warned of a New Labour stitch-up.
Mr Balls showered praise on the Lib Dem Cabinet member today in an apparent bid to exploit cracks in the coalition government, labelling him "a man to do business with."
Mr Balls told the BBC's Andrew Marr Show that he wanted a Labour majority government in the next election.
"But right now I want decent people who put the country first and are sensible in their thinking," he said.
"I wish George Osborne would see Vince Cable as a man to do business with and listen to, rather than telling the newspapers he is putting his allies in to try and surround him and hold him back," he said, alluding to newly appointed Tory junior minister Michael Fallon.
The charm offensive began in a Sunday Mirror column urging Mr Cable to look into his "heart of hearts."
Mr Cable played hard to get, saying he was "part of this coalition government and committed to working in it."
He added: "I do have perfectly businesslike, amicable relationships with members of the Labour Party and other parties. I see no problem in that - that is grown up and sensible."
The not-so-coy courtship drew sighs from the grass-roots Labour Representation Committee (LRC), which is hosting leading trade unionists including Unite's Len McCluskey at TUC Congress on Tuesday night to discuss how the unions can shift Labour left.
LRC secretary Andrew Fisher told the Morning Star he was not surprised but added that Mr Balls's strategy itself was far from sensible.
"It's the same old thing that just keeps coming up - the right of the Labour Party has always seen it as a way of excluding the trade unions."
Mr Cable was less overtly right-wing than his Cabinet colleagues, "but if that's described as sensible, then that's very worrying for the Labour Party.
"If Labour can't win without the right-wing in the next election, it's a sign that the working-class haven't the confidence in them," he said.