Santorum is #3 in the GOP Senate PEcking order (Frist, McConnell, Santorum). Santorum is Republican Conference Chairman, and it is nothing to shake a stick at.
It's based on seniority. Of course, they can ignore their own rules, but it won't happen, and it certainly won't happen because Santorum says so. If you want to boycott someone, I suggest that it be Snowe and Chaffee, both of whom are up for re-election in 06.