Before Biden exited the race, a Jul 2 poll by Reuters/Ipsos had Harris losing to Trump by 1 point. At that time, the only Democrat polled who beat Trump was Michelle Obama, who had an 11 point advantage over the former president.

https://ace-usa.org/blog/research/research-votingrights/could-joe-biden-be-replaced-as-the-democratic-party-presidential-nominee/