They only ramped up their opposition to him once he switched to run as an Independent.
'Freaked out' Democrats mobilize to stop RFK Jr. from cutting into Biden
"This year, though, Democrats have adopted a radically different strategy on independent and third-party candidates. They’re waging
an open war on the likes of Robert F. Kennedy Jr., whom they view as mortal threats to President Joe Biden’s re-election."
So their opposition to him was really just out of fear of him pulling votes away from Biden.