To counter everyone saying "they do," could you be referring to more high-profile cases?
Trump was shot at, but not killed. Would've made history if he was.
The US doesn't have political assassinations on the level of say, Mexico, but they are frequent enough for someone to be able to point to them still happening. Usually it's about some congressperson that we've never heard of.
I'm not sure if there's greater protection based on the popularity of politicians. It would make sense if someone like AOC gets more protection than, well, someone we haven't heard of.