According to the Federal Trade Commission website Antitrust Laws--Federal Trade Commission The Sherman Antitrust Act of 1890 outlaws any "monopolization, attempted monopolization, or conspiracy or combination to monopolize."
In a piece written for The Libertarian Republic , William L. Kovacs argues that Sherman and other antitrust legislation applies to the conspiracy created by the Republican and Democratic Parties against alternative party and independent candidates.
Kovacs gives a brief description of why antitrust applies and why common arguments against antitrust made by the two major parties are invalid. He outlines the main legal hurdle as showing how the major parties affect commerce, as most antitrust laws are concerned with restraint of commercial trade. A Conscious Conservative believes that with the current state of government meddling in markets, this should not be too difficult to show. The main hurdle will be that almost all judges are aligned with either the Republican or Democratic Party. I wonder how judges would feel about that as evidence that the two party system is an illegal conspiracy?