What happened in June? (he's just tweeted it yet again). In June wasnt the main story the virus and the abject state of the US's handling of it ? The killing of George Floyd did not help trump but thats not the main story now, it's this. He does appear to be making his whole re-election campaign around the protests / unrest. Which is kind of amazing when you look at it, the incumbent saying 'see this burning car these shootings this boarded up shop it is Biden's fault' just doesn't make any sense.
Trump took a hit (and Biden got a boost) in the polls throughout June, to what extent that was down to the poor handling of Covid-19 as opposed to the BLM protests I don't think you can disentangle (or even if it would make sense to, the combination of Trump handling both so badly was probably part of the reaction).
But Trump's making racial/sexual issues the battleground can come back to bite him - Clinton got a real boost in her lead after 'grab them by the pussy', his comment's around Charlotteville coincide with his period of greatest disapproval. Now part of effect of such things tends to wear off, it's hard to tell but the slight narrowing of the polls recently could equally (I'm my personal opinion more likely) be explained as a return to the average as support for anything Trump has said on rioters.
There's a lot of dislike of Trump's handling of racial matters, and there's evidence that the US electorate feels differently about race in 2020 than 2016 (
1,
2,
3,
4). There was a lot of support for the BLM protests, that's died back a bit (
video) but Trump going on a full on racist spree could easily see support bounce back.
I think it is too early to say that Trump is going to make this centre of his campaign (or if he even can do very possible to see an event like a 2nd eve dominating the campaign), more he's just reacting to something in the news at the moment. And it's possible that opposition to the protests grows and benefits Trump, but I don't think that's nailed on.
EDIT: And even if the protests do play badly with some voters that not to say that it is necessarily in Biden's favour to go all out denouncing them. The whole electorate is so polarised that Biden may do better to show limited support for the protests, giving the Democrat-leaning base a reason to vote for him, stress his progressive credentials a bit.