He later served as Executive Secretary and National Chairman of the Social Democratic Federation, a factional offshoot of the Socialist Party.

He also was a frequent speaker at public meetings of the Workmen's Circle (Arbeiter Ring), a predominantly Yiddish-language fraternal benefit organization.

He also taught extension classes in Labor and Management for Rutgers University and was a volunteer speaker and organizer for various New York trade union locals.

[5] Early in his tenure as a teacher at the Rand School, Claessens met a student named Hilda Goldstein, who he subsequently married.

Gus Claessens made his first run for political office in 1914, when he stood for United States Congress as a candidate of the Socialist Party in 1914.

After the youthful radical "Militant" faction won the day at the 1934 National Convention, passing an aggressive new Statement of Principles, Claessens joined his "Old Guard" comrades in an exodus from the party to establish the Social Democratic Federation (SDF).

[3] In the middle 1930s, Claessens served as Executive Secretary of the SDF and was later elected to the position of National Chairman, a post which he held until his death in 1954.

[3] Claessens was active in the American Labor Party during its early years and stood for election to the New York Assembly as part of a joint ALP-Republican ticket in 1938.

[11] In 1944, Claessens joined with Socialist Party leaders Harry Laidler and Paul Blanshard in attempting to defeat the Communist-dominated faction which had gained control of the ALP.