The intention was to spread panic and paralyze the postal service.
I have no idea how such a diverse group of targets came to the attention of the people behind this - remember, the NY Post received a letter, and the Post is something that makes liberals shriek in horror and dissolve. And the National Enquirer is so far out in outer space that normal political standards don't apply. (Btw, it is thought that the first letter did have spores that could produce inhalation anthrax - therefore, not unsophisticated at all.)
I don't think you can line it up on left/right grounds. To me, it seems like this would be the action of somebody who has been here for a long time, and knows a lot about the country - but maybe doesn't quite get it. However, knowing about somebody as obscure as Leahy might indicate some sort of geographical connection or, depending on what committees he had worked on or bills he had supported, some sort of issues-related connection that might give a clue to the identity of the terrorist.