Lib Dem MP Adrian Sanders, a former aide to Paddy Ashdown, said the deputy prime minister needed to stop "just bumbling along worrying about the future" and take action to rebuild support for the party, he told the Sunday Times.
His comments were echoed by the Lib Dem peer, Lord Smith of Clifton, who warned Clegg he was not "indispensable" and could be replaced by the business secretary, Vince Cable.
Smith, a former politics professor, is the second Lib Dem peer in a week to question Clegg's leadership.
He told the paper the deputy prime minister was "just a cork bobbing on the waves" with "no strategic vision at all".
"It's not as if Clegg is indispensable. Vince Cable possesses the appeal and the credibility to lead the Liberal Democrats into the next election," he said.
Andrew Bridgwater, the vice-chairman of Devon and Cornwall regional party and chairman of the Lib Dem education association, went further, calling on Clegg to depart. "The sooner Nick resigns and creates a vacancy for Vince, the better," he told the Independent on Sunday.
"To put it bluntly, I would encourage Vince Cable to stand for the leadership to take us into the next election."
Senior activists have accused Clegg of failing to stand up for the party's interests on tax, education, electoral reform, the NHS and Europe.
Another activist, Charles West, the chairman of Shrewsbury and Atcham Lib Dems, told the paper Clegg must "justify" his decisions in government over the next two years or be at risk from a leadership challenge.
The comments come as a YouGov poll showed 47% of Lib Dem voters think the party would fare better if they ditched Clegg before the next general election.
Former Liberal Democrat voters, who have abandoned the party since it went into coalition with the Tories in 2010, are most strongly in favour of dropping Clegg, with 63% saying the party would do better without him.
Last week, one of Cable's closest allies, Lord Oakeshott of Seagrove Bay, warned the Lib Dems may need to consider a change of "management and strategy" because the party had seen its support halve since the election.
Oakeshott, a persistent critic of Clegg, advises Cable on an informal but regular basis but his remarks prompted warnings by senior Lib Dems that the business secretary is one of two cabinet ministers making plans to succeed Clegg. Another source said Ed Davey, the energy and climate secretary, is also manoeuvring to oust Clegg, but the MP himself has dismissed the claim.
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