The Missouri Compromise occurred in 1820. It set the idea that all states North of the 36. 30 parallel would be free and all below would be slave states (minus Missouri). It was an attempt to contain the spread of slavery.
The answer to your question is the lobbyists. They are the people who represent interest groups to legislators. The lobbyist's role is to try to influence a government official's decision specifically legislators or the members of the regulatory agencies.