Green’s Dictionary of Slang

(N.Z./US) a public orator, esp. an extremist or rabble-rouser.

1910		Hawaiian Gaz. (Honolulu, HI) 1 Nov. 6/2: If the soapbox artist could talk Chinese as well as rolls out English.
1919		Dayton Dly News (OH) 3 Oct. 19: It’s all wrong to stand idly by and see our institutions atacked by those ungrateful soapbox artists.
1927		jrnl News (Hamilton, OH) 26 Jan. 5/4: These soapbox artists preach in inflammatory language.
1938		R. Hyde Nor the Years Condemn 267: That’s only the Reds gabbing. Soapbox artists, we call them.
1944		Austin American-Statesman (TX) 2 Dec. 4/1: ‘What we need are more soapbox artists spouting on public squares’.
1986		Newark Advocate (OH) 28 Sept. 9 /5: The other man didn’t answer, which gave ye old soapbox artost another chnace to spout off.
1998		Dly Chron. (De Kalb, IL) 26 Jan. 4/6: Is there a microphone in America that this soapbox artist has not yet declaimed before?
2004		L.A. Times 11 Sept. E6/5: We meet Chicago’s aging ‘soap-box artist’, Carlos Cortez, a conscientious objector during World War II.