...but if it helps, you made a little English error. Wouldn't even have mentioned if you hadn't said you were French :)
> [Republicans.] Most of which are either moderates or closer to the Democrats than to their own party.
> Olympia Snowe and Susan Collins are the two Republican Senators from Maine which are both regarded to be leading moderates within their party.
Should be "Most of whom..." and "...who are both...".
"Which" is only ever[1] used in relative clauses to describe inanimate objects (as you do correctly five other times in the article). Humans and animals are always either "who" (more personal) or "that" (more impersonal).
Note that's only in relative clauses-- "Which senator is yours?" is fine, but "The senator which represents me" means that I think my senator is a card table.[2]
[1] Outside of the odd dialect, of course.
[2] Not necessarily out of the question.