The Liberal Democrat leader said he would "not be confident at all" entering into such an arrangement in 2015 with a party that "doesn't understand that breaking the bank is a deeply regressive thing to do".
But Mr Clegg appeared to leave the door partly open by adding: "Look, if the British people were basically saying 'the only way in which you can govern this country is with Labour,' then of course, I'll try and see if that's possible. It might not be possible. Who knows?"
The Lib Dem leader claimed Ed Miliband's party still believed that "good policy is synonymous with a big spending policy".
He told Prospect magazine: "In fact, I just don't see any evidence that Labour understand what it is to be a post-2008 progressive."