The best-known of his publications, Capper's Weekly, had an enormous readership among farm families and served as the base of his political support in Kansas.

He was in the Senate from 1919 to 1949 and was prominent among Republicans who supported the relief efforts and other policies of Franklin Delano Roosevelt's administration.

In 1923 Senator Capper brought forward a constitutional amendment with an anti-miscegenation provision outlawing mixed-race marriages, but struck out the passage after protest from African-American organizations and stated it was an unnecessary troublemaker.

[3] In April 1943 a confidential analysis by British scholar Isaiah Berlin of the Senate Foreign Relations Committee for the British Foreign Office described Capper as: a solid, stolid, 78-year-old reactionary from the corn belt, who is the very voice of Mid-Western "grass root" isolationism.

Like Johnson and Nye, an unwavering opponent of all the Administration's foreign policies, including reciprocal trade.

[4]Capper became chairman of the Senate's Agriculture Committee in 1946; by that point, at the age of 81, he was nearly deaf and his speech was difficult to understand.

[5] He joined the Congressional Flying Club in 1947 at the age of 82 and took up flying lessons, as the oldest member of Congress, from Pearle Robinson, part owner of the Hybla Valley Airport just outside of Washington, D.C.[6][7] After retiring from the Senate, Capper returned to his home in Topeka, Kansas, where he continued in the newspaper publishing business until his death.