That's just simply not true.
IAPolls ran Trump v Biden, then immediately ran Trump v Biden v RFJjr, against the same set of battleground states.
Whereever Trump led versus Biden, he still leads with RFJKjr added, including a statistical tie with Biden in MI.
In fact, with RFKjr added, Trump advances into a statistical tie with Biden in PA.
"Kennedy's deep-pocketed backers and famous name, combined with a lack of broad enthusiasm for Biden and Trump, could help him take votes from their respective sides."Our concern about all third parties is that Donald Trump’s support is limited – he’s well below 50% - but very stable," said Matt Bennett, the co-founder of Third Way, a left-leaning political strategy firm. "Voters who would reluctantly pick Biden in a head-to-head with Trump might jump at the chance to vote for a Kennedy, even if they don’t know much about him."