A friend in the US wrote this, not saying I agree but she is a very "on the ball" Democrat:
"Actually her (Harris)most recent ratings have been better than either Biden’s or Trump’s, and as other recent polls have suggested that pretty much any Democrat, other than Biden, has a very good chance of beating Trump, I wouldn’t write her off.
It’s not a slam dunk that it will be her, though, in theory the delegates at the convention could choose anyone,
and one of the suggestions is to hold an open contest during it, allowing anyone who wants to throw their hat into the ring to make their case, and for the delegates to vote accordingly.
OTOH everyone might well get behind Harris who, it has to be said, is mainly unpopular in certain quarters, whose voices tend to be heard the loudest (eg Fox News) because she’s not male, or white.
A lot would ride on her choice of running mate, and there are a lot of good candidates.
One interesting one, which I hadn’t thought of until I read an article the other day, is Mark Kelly, senator from Arizona. He’s popular, has national name recognition, his wife is a survivor of a mass shooting/assassination attempt, and he has patriotic credentials up the wazoo (navy fighter pilot and astronaut) which would appeal to independents and moderate, never-Trump Republicans.
It wouldn’t risk a Democratic seat in the senate either, as the governor of Arizona (a Democrat) would get to appoint someone to serve out the rest of his term.