Travis Bickle’s attempt to assassinate Senator Charles Palantine is an integral part of his downfall in Taxi Driver, but his motivation for the plan isn’t entirely clear. Palantine’s presidential campaign is a running thread throughout Taxi Driver. Travis’s love interest, Betsy, is a volunteer working on the campaign, and he first asks her out — and later furiously confronts her — at the campaign office. At one point, Palantine even rides in Travis’s cab when he’s in too much of a hurry to wait for his limo. As his sanity deteriorates and the “bad ideas” in his head bubble to the surface, Travis decides to kill Palantine.
https://screenrant.com/taxi-driver-travis-assassinate-charles-palantine-reason/