Berger was the first Socialist elected to the U.S. Congress, and served as U.S. Representative for Wisconsin's 5th congressional district from 1911 to 1913, and from 1923 to 1929.

He co-founded the Socialist Party of America in 1901.

• After being convicted under the Espionage Act for opposing U.S. involvement in World War I, he was twice denied his House seat until the Supreme Court overturned the conviction in 1921.