Experts like Sean Wilentz at Princeton say that third parties pioneered the abolition of slavery, introduced the concept that every adult should be able to vote, led in working for women’s right to vote in the 18-1900s, advocated laws against abusive child labor in 1904 and with the Populist Party, introducing the 40-hour work week, which led to the Fair Labor Standards Act of 1938, advocated for the 8 hour work day, paid vacation, maternity leave and many other things.
Adam J Sacks has a PhD in history from Brown University and writes about how it was socialist worker movements that pioneered your very right to even vote. He explains how the first protests for voting rights for all men (not limited to those with property) originated from socialists in Belgium in 1893,1902 and 1913 and then spread to other countries and eventually included women. So you can thank socialists for your freedom to have a voice in elections.
“Contrary to the mainstream story that capitalism naturally gave rise to democracy, establishment powers in nineteenth-century Europe restricted the vote for as long as they possibly could. Only when faced with mass mobilisation — or when continent-wide war wiped out working-class males en masse — was it clear that the franchise could no longer be withheld. The particulars of individual European countries varied. In some nations, following intense struggles, workers won limited forms of universal male suffrage before World War I. More commonly, broad suffrage rights appeared only after the war. But what was consistent were the actors pushing for universal suffrage: trade unions and, crucially, socialist parties. In fact, what has been called the “democratic breakthrough” of the nineteenth century could easily be called the “socialist breakthrough.””
See also: https://www.constitutionparty.com/a-brief-look-at-the-history-of-third-parties-in-america/, https://news.virginia.edu/content/third-party-impact-american-politics, https://www.sparknotes.com/us-government-and-politics/american-government/political-parties/section3/)
“What happens is third parties act as a gadfly,” said Sean Wilentz, director of the American Studies program at Princeton University. “There’ll be an issue that’s being neglected or that is being purposely excluded from national debate because neither party wants to face the political criticism that it would bring. A classic example was slavery.”