Final answer:
Thomas Nast, a 19th-century cartoonist, popularized the association of the Republican Party with the elephant and the Democratic Party with the donkey.
Step-by-step explanation:
The associations of the Republican and Democratic parties with the elephant and donkey, respectively, were popularized by Thomas Nast, a cartoonist for Harper's Weekly in the 19th century. One notable cartoon, titled 'Stranger Things Have Happened' from 1879, encapsulated this symbolism. These animal symbols were likely chosen as a way to represent the characteristics perceived to be associated with each party during that time and have endured as a political shorthand for the parties.