Final answer:
The most significant third political party of the late nineteenth century was the Populist Party. Formed in 1892, the Populist Party called for reforms such as railroad regulation and the popular election of U.S. senators.
Step-by-step explanation:
The most significant third political party of the late nineteenth century was the Populist Party. Formed in 1892, the Populist Party emerged as a response to the perceived dominance of big business and the decline in power of farmers and rural communities. The party called for railroad regulation, an income tax, and the popular election of U.S. senators. Although the Populists supported the Democratic candidate in the 1896 presidential election, they ultimately disappeared from the national scene after 1908.