The quote:
“Given Joe Biden’s incredible record, given Donald Trump’s terrible record: he should be mopping the floor with Donald Trump. Joe Biden is running against a criminal. It should not be even close. And there is only one reason it is close. And that is the president’s age.”
A Jul 2 poll conducted by Reuters/Ipsos had Kamala Harris losing to Donald Trump by 1 point (42% to 43%) if she were to replace President Biden. The only Democrat who would hypothetically beat Trump according to the poll is Michelle Obama, who would have an 11-point advantage over the former president. However, the former First Lady has expressed several times over the years that she will not be running for president.
https://ace-usa.org/blog/research/research-votingrights/could-joe-biden-be-replaced-as-the-democratic-party-presidential-nominee/