Not “over” in the sense that he won’t hold another event before November, “over” in the sense that his chances of a comeback realistically are now up in smoke. That’s the claim being made today by various observers, and not just Democrats.
I’m inclined to disagree although a strong counterargument eludes me. Partly I’m skeptical because of the lingering memory of the “Access Hollywood” episode, which happened at around this point of the 2016 race and was also cracked up at first to mean the end for Trump. And his polling did take a hit in mid-October — before rebounding steadily over the final three weeks of the race, abetted by James Comey’s announcement that he was reopening the Emailgate investigation. But there’s no real logic behind the comparison besides a facile feeling of “this setback vaguely reminds me of that other setback, and he recovered from that other setback, so why wouldn’t he recover from this one?” Other people have thought of the “Access Hollywood” scandal today as well, although their logic doesn’t seem to be any better than mine:
Source close to Trump campaign just likened Trump's positive covid-19 test to the fallout after Access Hollywood tape was released on Oct. 7, 2016. It (temporarily) cost him ~ 2-4 points in public polling. "I'm guessing this is slightly more problematic than that," source added.
— Jacqueline Alemany (@JaxAlemany) October 2, 2020