Senator Stern is a recurring antagonist in the Marvel Cinematic Universe, serving as a major antagonist in Iron Man 2 and a minor antagonist in Captain America: The Winter Soldier, where he was revealed to be a member of HYDRA.
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. After Tony and Rhodes defeated Ivan Vanko and Hammer, Tony requested that Stern personally give medals and commendation to Tony and Rhodes, 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." It turns out that Stern and Sitwell (along with the Winter Soldier, Brock Rumlow and the STRIKE team) are involved in Alexander Pierce's true plot to utilize the Hellicarriers and Arnim Zola's algorithm in taking down millions of people who are opposed to HYDRA in an attempt to bring in a new world order for HYDRA.
However, Black Widow and Nick Fury uploaded the HYDRA's secrets after Captain America exposes the plots, exposing Stern and all of the other secret HYDRA agents. The last we see of Stern, he is being approached and handcuffed by two FBI agents, indicating that he will almost certainly be facing an indictment, and tenure in prison for his crimes.