Turns out it’s a 4th of July movie (which I watched on the 6th).
Months after a senator is shot by waiters at a Space Needle party, a reporter at the scene tells colleague Warren Beatty that witnesses are being killed. Warren goes rogue, raises hell with a small-town sheriff’s department and comes away with some papers from an organization that recruits assassins. He goes undercover calling himself Harry Nilsson, gets a crazy guy to fill out the admission forms for him, and convinces the parallax group he’s crazy enough to kill for them. I didn’t follow all the twists when he saved a flight full of people from a bombing, but the company is onto him afterwards, sending his editor a heart attack sandwich then framing Warren for the climactic assassination.
Alan “The Count” Pakula was the 70’s conspiracy thriller guy, making this between Klute and All the President’s Men. The screenwriters also did Three Days of the Condor (sure) and The Money Pit (what) with uncredited work by the late Robert Towne. They got a Taking of Pelham hijacker in this, a Stepford wife, and a Hitchcock actor.
Proof that it’s a 1970s movie: