This is good stuff (courtesy of US News and World Report's Washington Whispers blog: apparently the operatives at the RNC are following good language discipline and are not permitted to refer to "Barack" and "Hillary" but rather only "Senator Clinton" and "Senator Obama."
If people are used to calling a politician by their first name, it evokes a friendlier, warmer relationship than calling them by their title and last name.
That's one small reason why Arnold does so well in California. (I usually write Arnold because I'm too lazy to type out Schwarzenegger). Campaigns should cultivate the habit of referring to their candidates by their first name.