Kennedy was a democrat, and that was that.
What's changed is that the party he belonged to has moved so far leftward as to be all-but unrecognizable to what it was then.
From democrats... to modern-day democrat/communists.
What's discouraging is that the Republican party has moved leftward, as well, to the point where many of them occupy the mushy center/left position.
Consider that Susan Collins holds positions today that Kennedy and many of his contemporaries would have denounced as subversive and obscene.