Former minister Jeremy Browne said that the Libs Dems were heading for heavy losses at the election in May and could poll so badly in "90% of seats" that it would be "very difficult to recover" there.
In an interview with The Independent, he blamed Clegg's attempts to differentiate the party from their Conservative coalition partners and predicted the party would not seek another coalition in the next parliament, saying the Lib Dems were "collectively relieved that the whole ordeal is over".
"We are in no man's land. We don't have a distinctive proposition. We have sought to distance ourselves from a successful creation which has required a huge amount of effort and sacrifice by us to make it successful. That is the sad irony of our position," he said.
Browne, who is standing down at the election, said the Lib Dems "lost their way" as they tried to go from an opposition party to a governing one.
"We lost our way on that journey. We have gone from being a party of protest in opposition to often sounding like a party of protest in government," he said.
Jeremy Browne (right) with Nick Clegg in April 2013, in happier times, when Browne was a Home Office minister
"Nick Clegg made a decision that he was going to look inwards and talk to his party and reassure it, rather than look outwards and talk to the country and appeal to it."
He predicted that the Conservatives could emerge as the largest party after May 7 but said that he believed that the Lib Dems would reject the chance of joining them in a second coalition at which point Clegg would resign as leader.
"Being leader of a smaller party would feel unfulfilling and there would be a sense that his era had come to a natural conclusion," he said.
He forecast that Tim Farron, the Lib Dem foreign affairs spokesman, would be elected leader and take the party into opposition.
Browne, whom Clegg sacked as a minister in 2013, said his position became "more exposed" as the party turned away from the coalition and "thinking like a governing party".
He added: “I wanted the Lib Dems to really embrace government, to co-own it and psychologically be part of it. Not just having red boxes and ministerial cars but thinking like a governing party and shaping the agenda. That was the spirit of the party and its leader in the first half of the Parliament. It became less fashionable. As a result, I became more exposed.”
He even called the coalition "a winning proposition," adding: "If it was on the ballot paper, it would win in May. If people had a binary choice between continuity and change, I think they would vote for continuity. The mistake the Lib Dems have made, given we were instrumental in creating the winning proposition, is to distance ourselves from it."