By mentioning RFK Jr, he is keeping him in the news cycle to further fracture democrats, driving more voters (those who are diehard Democrats and wouldn't vote Trump regardless) away from Biden to Kennedy. If he never mentioned Kennedy, the democrats would bury him and pretend he doesn't exist. This way Trump saps traditional democrat support away from Joe, giving Trump a bigger victory margin.
By mentioning RFK Jr, he is keeping him in the news cycle to further fracture democrats, driving more voters (those who are diehard Democrats and wouldn't vote Trump regardless) away from Biden to Kennedy. If he never mentioned Kennedy, the democrats would bury him and pretend he doesn't exist. This way Trump saps traditional democrat support away from Joe, giving Trump a bigger victory margin.
This is what I beleive also, if Donald Trump says no - the never Trumpers will say yes which takes votes from Joe.