1780, in reference to members of U.S. Congress, and it first appears in a piece of abuse (written by a Loyalist):
Ye coxcomb Congressmen, declaimers keen,
Brisk puppets of the Philadelphia scene ...
Technically of members of both the Senate and the House of Representatives, but typically meaning only the House members. Congresswoman attested from 1918 (Jeannette Rankin (1880-1973) was the first).