What was Thomas Nast's impact on the American political system?
Nast was the first superstar political cartoonist.
You wouldn't know it today, but once upon a time, editorial cartoonists were a major draw for American newspapers and Thomas Nast was one of the first important ones. Nast was the in-house editorial cartooonist for Harper's Weekly from 1859-1886 and his illustrations contributed to the electability of Abraham Lincoln and Ulysses S. Grant. Today, he is most famous for his criticisms of New York City politics.
Nast's political cartoons added to the culture in subtle ways. The Democratic donkey and Republican elephant were his innovations. So was the popular version of Santa Claus (although Montgomery Ward's advertising agency, and Coca Cola's, codified it a little bit.)
Nast's greatest achievement was probably identifying Boss Tweed and Tammany Hall as corrupt institutions in the popular imagination of Victorian New Yorkers. Only Herblock and his unshaven Richard Nixon come close in terms of raising public awareness of arcane political issues.