Who was the 3rd party candidate that earned a significant number of popular votes?

Teddy Roosevelt, Progressive, 1912 (88 Electoral votes)

In the 1912 U. S. Presidential elections, former President Teddy Roosevelt emerged as the most successful third party presidential candidate in the history of the country when he bagged 88 Electoral votes and 27% of the popular vote in the election on behalf of the Progressive Party of the United States. The party was formed by Roosevelt himself when he failed to receive the nomination from the Republican Party in the 1912 Elections. However, Roosevelt lost, and the election was won by the Democratic Party's nominee, Woodrow Wilson, who went on to become the 28th President of the United States. The 1912 Presidential elections were unique in the fact that this was the last election where a candidate who was neither Republican nor Democrat came second in the election. This occurred as Teddy Roosevelt defeated Republican William Howard Taft and Socialist Eugene Debs.