This WPA poster was made to advertise a theatrical play from the mid-30s. "It Can't Happen Here" is a prophetic semi-satirical American political novel by Sinclair Lewis published in 1935.
A charismatic and power-hungry politician is elected President of the United States on a populist platform, promising to restore the country to prosperity and greatness, and, more importantly, promising each citizen five thousand dollars a year. Once in power, however, he becomes a dictator; outlawing dissent, putting his political enemies in concentration camps, and creating a paramilitary force called the Minute Men who terrorize the citizens. One of his first moves as president is to make changes to the Constitution which give the President sole power over the country, rendering Congress obsolete. This is met by protest from the congressmen as well as outraged citizens, but a state of martial law is declared and, with the help of his Minute Men, the protesters are thrown in jail. As democracy is dismantled, most Americans either support him and his Corpo Regime wholeheartedly or reassure themselves that fascism "can't happen" in America (hence the book's title).
It has been redone several times over the years with the most recent incarnation being the TV series "V".