The June 2006 Journal of Politics carries a political science article titled, “The Decline of Third Party Voting in the United States.” The authors are Professor Shigeo Hirano of Columbia, and Professor James M. Snyder of MIT. The article can be read here.
The article covers the period 1890 to the present. The title is misleading. The article says that third parties polled high shares of the vote between 1890 and 1920, and then low shares 1940-1970, and since 1970 it has risen again. The article seems not to discuss the period 1920-1940. The article is somewhat confusing because the authors never say whether they are including independent candidates in their study. They use the terms “two party system” and “third party” but don’t define either one.
The article introduces the idea that ballot access barriers may be responsible for the decline between 1920 and the period 1940-1970, but then they shy away from trying to determine if ballot access barriers are responsible for the decline. They say, “Ballot access restrictions varied across states over time. The information on specific ballot access restrictions is not readily available and consequently is not used in our analysis.” Of course, it is readily available, and was published in Richard Winger’s article in the Election Law Journal, vol. 5, no. 2. Appendix “F” gives the number of signatures needed for new party or independent candidate ballot access, for each state, for the entire period starting in 1892. But Professors Hirano and Snyder were apparently unaware of that resource.
UPDATE: the authors advise that they did include independent candidates in their compilation.