Railroads required enormous capital investment and became a natural monopoly as did steel and many aspects of power production. New regulatory schemes were invented to preclude such an amalgamation of power.
“Section 1 of the The Act characterizes certain business practices as a per se violation. Section 2 of the The Act prohibits monopolization, attempts to monopolize, and conspiring to monopolize (http://www.law.cornell.edu/wex/antitrust).” The first section bans anti-competitive behavior whereas the second section bans monopolies and they are meant to work in tandem.
When you think about trust busting, you probably think about steel or railroads in the 1920s. But how are modern political parties any different? Go back and read the first two paragraphs; both apply equally to major U.S. political parties in the 21st century. Could it be that the two party system, formally supported by most Secretaries of State, violate the various anti-trust laws in place in the United States? After all, the parties are set up formally to create immense barriers to entry (of the electoral process). Enormous capital investment is required to compete in the political arena creating insurmountable barriers to entry. There clearly is an oligopoly with the two party system buttressed by state law that excludes competition on the election field.