Until the late 60's/early 70's, the majority of liberal Democrats were patriotic and anti-communist. There was no doubting the patriotism of Truman, JFK, etc. however much we may disagree with their specific policies.
However, while I don't doubt that JFK was patriotic and well-intentioned (i.e. better than any Democrat and many Republicans today) I have yet to be convinced of his greatness. Seems to me that his alleged greatness was mostly due to media hype bordering on cult of personality - the "Camelot" myth when he was alive, the martyrization after his assassination.
Whether or not one thinks JFK might have achieved greatness with a full 8 yrs in office, he was cut down before he achieved any greatness. It’s all myth making.