I think you misunderstand the concept of a mandate. It is used when a party wins an overwhelming majority in the lower house to suggest that the upper house, where the party has a minority, should capitulate on a particular issue. I.e. suggesting the checks and balances should step aside. It has nothing to do with complaining about losing in a democracy.
Personally I think a mandate almost never exists. In a representative system you might say that all elected members have a mandate to vote as they campaigned.