Kevin de Leon is running against her and has some political and media support. Unfortunately, he's far to the left of her on the issue of gun control and that alone may nullify whatever small chance he had of getting elected.
Senate races are statewide and so districts are irrelevant.
If he's running for US Senate, all of California will be voting for him or Feinstein. And parts of California are extremely pro gun, and parts are extremely anti gun.