Appealing to soft Conservative voters is "electoral suicide", Matthew Oakeshott has warned.
The Liberal Democrat peer, a close ally of business secretary Vince Cable, hit out against the approach of Nick Clegg's recently departed director of strategy, Richard Reeves, in a fringe event at the party's autumn conference in Brighton.
"That section of the country which is fed up with Labour not being radical enough - we must get some of that back," he urged.
"We clearly are not going to get all of them back, but the Richard Reeves strategy that we've lost three or four million voters, we write them off and we target a million soft Tories in the middle - that's not a strategy, that's electoral suicide."
Oakeshott said that 39 out of the Lib Dems' 59 seats won at the 2010 general election were taken against the Conservatives - but that in many of these constituencies Labour votes had been squeezed historically over many decades.
The outspoken peer is one of the bolder of party leader Nick Clegg's critics, having suggested this summer that the coalition's junior party should consider dumping Clegg before the next general election.
He emphasised the importance of pushing for a mansion tax as a key strategy to win over uncertain voters ahead of 2015.
"[A mansion tax] would be a very clear Liberal Democrat win," he added. "We need something really specific we can say to people on the doorstep that we've achieved in government."
Oakeshott avoided repeating attempts to undermine Clegg. When asked about the leadership question he proposed that VAT be cut from 20% to 15% and made a quick exit.
Party president Tim Farron, who also spoke at the event and is viewed as a potential successor to the current deputy prime minister, acknowledged that the personality of a leader is becoming more prominent in voters' minds.
In 2010 it is as significant to voters in determining who they vote for as the party's policies, Ipsos Mori's Ben Page said.
Farron predicted that Labour would be let down by its leader, Ed Miliband, at the next general election.
"What costs leaders at general elections... is not their personality. It's whether people can envisage them in No 10 and in government," he explained.
Labour's Neil Kinnock and Michael Foot, and the Conservatives' William Hague, were all cited as examples of leaders about whom voters would conclude:" I can't see that feller in No 10."
He acknowledged that Clegg had taken a "personal hit" in the polls but insisted voters would conclude: "I can see that feller in No 10, because he's doing a good job in government."