They cannot.
The republican party is complicit, and supports the evil he does, and more than that, he commands their supporters, so it would be political suicide for them.
Why can’t the democrats? The republicans control all 4 parts of government and even if they had control over the senate, they would need 60% of the votes, a number that is unprecedented in recent times.
Assuming UK cops aren’t as utterly as untrustworthy as north american cops, (which you probably are purely because you don’t always bring around a gun), what are you meant to do with a knife attack? Wack people with a stick? Run away and called for armed backup? I assume the latter, but what about in the immediate?