Ironically, this could make him more popular. The establishment is trying to shut him down. That crap appeals to people that like RFK Jr.
But Trump isn't "establishment." Even though he controls everything about one of America's political parties...
His worshipers feel that way, but I'm seeing a lot of aimless, conspiratorial anti-establishment men talking about Trump and Biden as though they're the same. This is good for Biden. Trump needs those voters. Evangelicals didn't win 2016 for Trump, they've been voting straight-ticket Republican forever. It's these reluctant "shy" Trump voters that he needs and RFK Jr. is appealing exclusively to them.