Jeremy Browne said a substantial number of LibDems were happy for the party to be viewed as a "peripheral force that campaigns against the Conservatives".
But he warned they should be fighting to win back popular support after years of dismal poll ratings rather than focusing on existing supporters.
In an interview with The Times, Mr Browne admitted that being fired from his Home Office ministerial post by the Deputy Prime Minister was puzzling and painful.
"I was a very early and enthusiastic supporter of Nick Clegg," he said.
"You can't be half in and half out of government. We have to try to avoid the trap of looking like a party that is a reluctant party of government and looking uncomfortable and that we'd be grateful to be relieved of our collective responsibilities."