The two parties said on Wednesday that they would be against Croatia's entry into NATO if a decision to that effect was not confirmed at a referendum.
They agree that Croatia's entry into the alliance is in its economic, political and cultural interest, but that it is Croatian citizens who should decide on the matter at a referendum and that their wish has to be respected.
Two top Croatian officials - Parliament Speaker Vladimir Seks and Prime Minister Ivo Sanader - have said there is no need for holding a referendum on the matter.