Lord Oakeshott - a close friend of Business Secretary Vince Cable - resigned from the party last week after admitting he had commissioned research on how it would fare under a different leader. In a parting shot, he warned that the Lib Dems were heading for "disaster" with the Deputy Prime Minister in charge.
"What has happened after these elections has made a bad situation worse," Lord Ashdown said.
"We now know there has been a plot of deep malice to try and remove the leader.
"I have got a very clear message for the party, and that is anything you do now which is not getting out on the streets campaigning in the context of the next general election - which is where we can put our proposition of how we have behaved in government behind a fine leader and I think a fine record - is a distraction and a dangerous distraction."
Asked if he thought Mr Cable was involved in the plot, Lord Ashdown replied: "He says not, Nick says he wasn't."
However, he referred to Shakespeare's Othello - who was betrayed by his confidante Iago.
"One of the rules of politics is choose your Iagos carefully," he said.
According to a poll at the weekend, Mr Clegg has become the least popular party leader in modern British political history.
YouGov research found just 13% thought the Deputy Prime Minister was doing a good job - compared to 78% who said the opposite.