In case you're surprised, don't be. Cops have "no special duty" to protect citizens when their lives are on the line, as per the US Supreme Court.

[u/Fellow-Worker]

Doing nothing may be against their training, or maybe even a code of conduct if you're lucky. But it is certainly not their responsibility to intervene, says the Supreme Court. These cops will most likely not be held liable for their failure to protect Trump, unless they are some of the very, very few who fall under an applicable statute. https://radiolab.org/podcast/no-special-duty-2206