If you go further back in history, the other notable alternatives to the Democrats and Republicans were the Populist (or People’s) Party in 1892, Theodore Roosevelt’s Progressive Party in 1912 and Robert (Fighting Bob) LaFollette’s Progressive Party in 1924 and Henry A. Wallace’s Progressive Party in 1948.
All of these parties except Henry Wallace’s actually carried states. TR’s Progressives actually received more popular votes and electoral votes than incumbent Republican President William Howard Taft.
The most successful third-party and independent candidates—Theodore Roosevelt, George Wallace and Ross Perot—were celebrities before they ran.
The Populists definitely influenced the major parties. Democrats in 1896, 1900 and 1908 nominated William Jennings Bryan, who advocated most of their reform platform.
Theodore Roosevelt was not a spoiler. Public opinion in 1912 favored progressive reform, and Woodrow Wilson, the victor, probably would have received much of the vote that went to TR.
Henry A. Wallace, interestingly, received almost as many popular votes as Strom Thurmond. They each got about 2.5 percent (as did Ralph Nader in 2000). But, because Henry Wallace’s votes weren’t concentrated geographically, he didn’t receive any electoral votes (nor did Nader).
It’s noteworthy how few votes Thurmond needed to carry four states—Louisiana, Mississippi, Alabama and South Carolina. I wonder how much was due to apathy and how much to voter suppression. I read somewhere that in the 1928 election, the total votes cast in the former Confederate states were less than the voter turnout just in New York state.
Thurmond’s and George Wallace’s candidacies, along with Barry Goldwater’s candidacy in 1964, were part of the transition of the South from predominantly Democratic to predominantly Republican.
Ross Perot may have been a spoiler. Bill Clinton’s 1992 victory was narrow, and I think Perot took away more votes from George H.W. Bush than from Clinton. Perot’s emphasis on balancing the budget may have influenced Clinton, but his opposition to NAFTA most certainly did not.