I'm assuming because this is under the history thread, it's dealing with the U.S. Senate. There are two main political parties in the U.S. which are Democratic and Republican- but there is also an Independent party, just not as big. Representatives are people who represent a certain party in, I believe, the Senate. So when it says "proportional", it's most likely referring to the fact there is an equal amount of representatives who are under the two big parties- the Democratic or Republican.
