It's not a law but the two parties control the Presidential Debates with a bipartisan commission that consistently excludes third parties: https://en.wikipedia.org/wiki/Commission_on_Presidential_Deb...

The history of it is very interesting; essentially it was formed after the League of Women Voters, who used to run the debates, stopped running them because the system had become too corrupt: "the demands of the two campaign organizations would perpetrate a fraud on the American voter."