Why do political parties try to become as large as possible and broaden
their platforms?

The larger a political party is the greater it's influence. As a political party grows in size it makes it easier to recruit more members. Think about it, more people are likely to join a political party that's popular and that they've heard of before than an obscure one. The bigger the political party gets the more resources they gain through donations. They are also able to communicate their ideas to more people. If a political party has more resources they can use lobbyists and afford more political advertisements as well. In this way they gain strength and are able to maximize their popularity and the likelihood that any candidate they back wins a given election

Explanation:

answer; the confederacy could afford to move a large number of prisoners hundreds of miles but could not afford to fee and house the prisoners;