Nick Clegg should step down as leader of the Liberal Democrats in order to maximise the party's chances of saving seats at the next general election, Matthew Oakeshott has said.
The Lib Dem peer, a close ally of business secretary Vince Cable, has been escalating his criticisms of the deputy prime minister throughout the coalition period.
Now he has made a dramatic call for change at the top as Lib Dems prepare to travel to Glasgow for their annual autumn conference.
"Let's be objective - we have to accept that Nick's ratings are very poor and have been for a long time," Oakeshott told the House magazine.
"You've got to be frank that his ratings are down at levels which if you go back were only seen by Mrs Thatcher shortly before she left and Michael Foot. It's nothing personal, you've got to look at the facts."
Glasgow will be the penultimate autumn gathering of the Lib Dems before the 2015 general election, when the party is expected to struggle to hold on to the 23% vote share it secured in 2010.
Many party members will be frustrated by Oakeshott, who is a well-known agent in the proxy war between Clegg and Cable.
But some will sympathise with the view that the party could win, in Oakeshott's view, as many as 20 more seats with a change in polling fortunes.
"We need to face facts, there's quite a lot of complacency going on and self-delusion going on," he added.
"We are likely to lose seats. If we are on 15% we would hold 30 seats, if we are on 17% we would hold 40 seats - and if we are on 13% we would hold 20 seats…. It's very important to maximise our national share of the vote to give our MPs the best chance."
Earlier this week Cable praised Sarah Teather's attack against the coalition's immigration and welfare policies, which the former minister cited as reasons for her decision to stand down from parliament at the next election, as "eloquent".
The business secretary is viewed as a potential successor to the deputy prime minister, but party president Tim Farron is also thought to be in the running.
Farron presented himself as a potential partner with Ed Miliband if the 2015 election returns a hung parliament.
He told the New Statesman he views Miliband as "polite and nice", a member of the "soft left" of the Labour party and who is "instinctively a bit more pluralistic than most Labour leaders and a bit more internationalist as well".