I am not so convinced. I think people have formed preliminary opinions but are waiting to see the candidates unfiltered. That happens on national television during the conventions and during the debates.
By "unfiltered" I don't mean authentic. Everyone knows these are stage managed events. But they provide a basic gut check for people trying to pierce the fog of a media they do not trust and a political advertising system they know is broken. They want to see the candidates perform for an extended period and gauge whether they want this person running the show or not.
Nor do I think it will be a sudden break, unless something dramatic happens. But I expect to see some shifting between Labor Day and election day.