It seems that developers are ready to talk about how Parties will factor into the political landscape of the game, especially since this is a mechanic implemented due to play feedback.
In Victoria 3, a Political Party is an alliance between Interest Groups. They appear in countries that have Elections, and are absent in countries without them, differentiating the experience and mechanics of democracies from autocracies.
Interest Groups must be added or removed from government as a single bloc, which means that you’ll need to make strategic decisions about the makeup of your government and deal with the consequences.
The Liberal Party may for instance contain both the Intelligentsia and the Industrialists, who will both back your plans to move toward a free-market economy but diverge on the issue of child labor. Sometimes an opposition Interest Group might decide that it wants to join a Party currently in government.
In this circumstance, the Interest Group will leave their old Party and be marked as “wanting to join” their new party. This won’t force any Interest Group into or out of government, but if you decide to reform your government under these circumstances you must treat that Interest Group as being a member of the new Party, bringing them into or out of government as a single bloc.
Just as Parties ally together when you form your government, they also share their Momentum in Elections, which determines their votes. If for instance the fascist leader of the Petite Bourgeoisie joins the Conservative Party and is caught up in a political scandal, the whole Party suffers the electoral consequences.
On the other hand, as the Landowners begin to lose their political relevance in an increasingly industrialized economy they might cling to relevance by attaching themselves to the rising Fascist Party, which may bring them Political Power from the votes the whole Party gains in the Election.