I definitely agree that Trump (and the GOP generally) has had a far more difficult time defining Harris or Walz than they did with Hillary or even Biden, but, assuming that the DNC goes well, I think his last chance to maybe turn things around will be his first debate with Harris. Both that debate and the vice presidential debate will likely be their last chances to regain some momentum. If Harris or Walz has a poor performance then both right-wing media like Fox and Trump's campaign will be all over it. But if they both do well then it's hard to see another major event (barring an "October surprise" scenario, which I don't think is going to happen this year) that could inject some life into what has been a surprisingly lifeless Trump/Vance campaign thus far.