Vince Cable has been forced to rally to the defence of Nick Clegg after one of his oldest political friends called on the Liberal Democrats to consider a change of leadership.

As more Lib Dems questioned Clegg's position, the business secretary said he ignored criticisms of the deputy prime minister, describing them as "inevitable" during the mid term of a government.

Cable spoke up on Radio 4's The World This Weekend a few days after Lord Oakeshott of Seagrove Bay, an old ally from their days in the SDP, called on the Libs Dems to consider a change in "strategy and management". Clegg's friends told the Independent over the weekend that the "Continuity SDP" was to blame for the briefings against the deputy prime minister.

Cable said: "I don't give any time to these personal criticisms of Nick Clegg which are being made at the moment. It is inevitable in the mid-term of parliament when party popularities wane, when difficult decisions are being made. That's happened many times before.

"The Tories are doing it – David Cameron has come under a lot of fire in the last few weeks. Nick Clegg has got the same. He's sufficiently resilient to ride through this, I think."

Senior party figures are losing patience with Cable, who has been aware for some time that Oakeshott, his informal adviser, is agitating against Clegg. Senior party figures believe that the business secretary, who is said by fellow cabinet ministers to be "on manoeuvres", is aware of Oakeshott's general approach.

One well placed party source said: "It is a bit weird for a cabinet minister to have an informal adviser like that. Matthew Oakeshott does freelance a bit. That is the nature of it."

Cable appeared on the BBC after the Lib Dem MP Adrian Sanders told the Sunday Times that Clegg must stop "bumbling along worrying about the future". The Lib Dem peer Lord Smith of Clifton warned Clegg in the newspaper that he was not "indispensable" and could be replaced by Cable.

One senior Lib Dem peer told the Guardian that Cable's defence was "lukewarm at best". "He could have been a little more forthright in his defence. It was a bit wet," he said.

Sir Menzies Campbell, the party's former leader, told the Sky News Murnaghan programme that some Lib Dem critics of Clegg were failing to observe the bigger picture. "Perhaps Lord Oakeshott, and others, should get out of the cloistered comfort of the House of Lords and perhaps the seduction of the studio. What Liberal Democrats are doing in government is an essential part of the strategy … of course there will be differences of opinion.

"People say sometimes political parties are like coalitions – well, the truth is coalitions are like political parties, because you have differing opinions at different times," he said.

The continuing debate over Clegg's suitability as leader will concern the party's grandees as the conference season approaches. A YouGov poll last week found that nearly half of activists believed the party would be better with a new leader.

A number of Sunday newspapers carried criticisms of Clegg by senior Lib Dem figures. Smith, a former politics professor, said 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 the 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.

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.