The 1912 race showed that what a third party achieved was to defeat the conservative Republican, and it led to the election of the Democrat Wilson, who began the massive statist rewriting of the Constitution that moved the nation away from its Constitutional foundations. It did little, indeed, to produce any victories for TR’s more moderate progressivism.
We also can look at the 1948 election, in which Harry S. Truman faced not only a Republican, but a third-party white Southern revolt led by the “states’ rights” candidate Strom Thurmond, as well as a left-wing national candidate, former Commerce Secretary Henry A. Wallace, who many thought would triumph or at least lead to Truman’s defeat because of would-be support from the left-wing of organized labor and the support of African-Americans in the North. Wallace got on the ballot with his “Progressive Party,” as the Communists who ran and created the movement called their new party.
The result, as Wikipedia concludes, is as follows:
The key states in the 1948 election were Ohio, California, and Illinois. Truman narrowly won all three states by a margin of less than 1%. These three states had a combined total of 78 electoral votes. Had Dewey carried all three states by the same narrow margins, he would have won the election in the electoral college while still losing the popular vote. Had Dewey won any two of the three states the Dixiecrats would have succeeded in their goal of forcing the election into the House. The extreme closeness of the vote in these three states was the major reason why Dewey waited until late on the morning of November 3 to concede. A similarly narrow margin garnered Idaho and Nevada‘s electoral votes for Truman. Dewey countered by narrowly carrying New York and Pennsylvania, the states with the most electoral votes at the time, as well as Michigan, but it wasn’t enough to give him the election. Dewey would always believe that he lost the election because he lost the rural vote in the Midwest, which he had won in the 1944 presidential election; given the effect the dramatic drop in farm commodity prices in the fall of 1948, a year of record farm harvests, may have had on the political mindset of the rural vote that November, Dewey may well have been right.
Truman’s victory can be attributed to many factors: his aggressive, populist campaign style; Dewey’s complacent, distant approach to the campaign, and his failure to respond to Truman’s attacks; the major shift in public opinion from Dewey to Truman during the late stages of the campaign; broad public approval of Truman’s foreign policy, notably the Berlin Airlift of that year; and widespread dissatisfaction with the institution Truman labeled as the “do-nothing, good-for-nothing 80th Republican Congress.” In addition, after suffering a relatively severe recession in 1946 and 1947 (in which real GDP dropped by 12% and inflation went over 15%), the economy began recovering throughout 1948, thus possibly motivating many voters to give Truman credit for the economic recovery. 1948 was essentially a Democratic year, as the Democrats not only retained the presidency but recaptured both houses of Congress as well. Furthermore, the two third parties did not hurt Truman nearly as much as expected. Thurmond’s Dixiecrats carried only four Southern states, a lower total than predicted. The civil rights platform helped Truman win large majorities among black voters in the populous Northern and Midwestern states, and may well have made the difference for Truman in states such as Illinois and Ohio. Wallace’s Progressives received only 2.4% of the national popular vote — well below their expected vote total — and Wallace did not take as many liberal votes from Truman as many political pundits had predicted.