Senator Stern is a minor character in Iron Man 2 and later became a minor antagonist of Captain America : The Winter Soldier where he was revealed to be a part of HYDRA as well as their agent.
He is portrayed by the late Garry Shandling.
Iron Man 2
Senator Stern was in a televised hearing of the Senate Armed Forces Commitee with Justin Hammer and Tony Stark. Stern demands that Tony Stark immediately hand over the Iron Man suit to the government, but Tony knows Justin Hammer and Stern's secrets, so he refuses this demand, and hilariously humiliates Stern and Hammer on national television.
When Hammer called Ivan Vanko, Stern was playing golf with Justin Hammer.
Later Tony and Rhodey defeated Ivan Vanko, Tony requested that Stern personally give medals and commendation to Tony and Rhodey, humiliating Stern yet again.
Captain America : The Winter Soldier
After Stern left a hotel with Jasper Sitwell, they gave a hug to each other, and Stern uses this momentary closeness to whisper to Sitwell "Hail, Hydra."
Black Widow and Nick Fury uploaded the HYDRA's secrets, exposing Stern and all of the other secret HYDRA agents. The last we see of Stern, he is being approached by two FBI agents, indicating that he will almost certainly be facing an indictment, and tenure in prison rather than in the Senate.