Earlier quoted context omitted.
Yes, by one lone shooter without support in the background. If there is a determined team of assassins .. all it takes is one well placed shot.
Again, the Secret Service is there to protect against all threats and the US Military there for every single other threat above the Secret Service’s pay grade and scale. It’s the most dangerous and most protected job in America no matter what the POTUS is actively doing at any and every single moment.
They didn't do a great job in the Pennsylvania case - they screwed up in some obvious ways and got lucky, essentially.