A political party is an organized group of people who have the same ideology, or who otherwise have the same political positions, and who field candidates for elections, in an attempt to get them elected and thereby implement the party's agenda.
Political parties nominate candidates to run many levels of government including the national level, Congress, and the presidency; but, they nominate for state and local levels as well. They also coordinate political campaigns and mobilize voters.
The preposition in question is "on", and its purpose is to show you where the dog chewed. You can remove the preposition to form the sentence "The brown dog chewed the rawhide bone." You can also rearrange it to find the preposition, "onthe rawhide bone, the dog chewed" in what is commonly known as the Yoda technique.
