Originally elected as a Republican, he represented Minnesota's 5th congressional district for a single term between 1917 and 1919, and lost renomination in 1918 due to his opposition to American entry into World War I.

Ernest Lundeen was born on August 4, 1878, on his father's homestead in Brooklyn Township near Beresford, Lincoln County, Dakota Territory.

His father, C. H. Lundeen, was an early pioneer who was credited with the naming of Brooklyn Township and the establishment of the school and other local institutions.

[5] In 1936, the Farmer–Labor Party nominated Floyd B. Olson, the popular incumbent Governor of Minnesota, for the open United States Senate seat vacated by Schall's death.

The state central committee of the Farmer–Labor Party selected Lundeen to run in his place, and he won a landslide election over Republican former Governor Theodore Christianson.

Initially, his Communist sympathies remained strong: in 1936, then Senator-elect Lundeen addressed a meeting of the "Friends of the Soviet Union" at Madison Square Garden as Tovarishchi ("Comrades").

He had close ties to George Sylvester Viereck, a leading Nazi agent in the U.S. Viereck, after giving the Senator millions of dollars in bribes, often used Lundeen's office, and "sometimes dictated speeches for Lundeen, openly using the Senator's telephones to obtain material from Hans Thomsen at the [German] embassy."

[3] It was likely an isolationist public relations tactic, reminding Americans that the United States should not lend more assistance to a country who was already in debt to them.

[3] On June 14, 1939, Lundeen joined a civilian and press delegation aboard USS Hammann for its sea trials off Fire Island.

[3] In 2022, Rachel Maddow released a podcast series titled Ultra, which explored Lundeen's complicity in Nazi Germany's intelligence and propaganda operations in the U.S. during the 12 to 18 months immediately preceding America's entry into World War II.

At the time of his death, the FBI was investigating Lundeen's ties to George Sylvester Viereck, a top Nazi spy working in the US to spread pro-Hitler and anti-Semitic propaganda.

[13] Norma Lundeen tried to prevent that narrative by claiming that "no one wrote [her] husband's speeches" and threatening to sue one of the journalists who was reporting on it.