It is not a question of what the party's official or unofficial name is. It is a question of the correct way of titling encyclopaedia articles. If we put a definite article in one article, logically we have to put it in hundreds and hundreds of other articles (The Republican Party, The Australian Labor Party, The Church of England), which serves no good purpose and makes it harder for readers to find articles with the search device (try it and see). It should be a rule that article titles do not include the definite article, unless it is part of the title of a cultural product such as a newspaper (The Scotsman) or a book (The Grapes of Wrath).