If it's the truth, or can be reasonably advanced as the truth based on the evidence, it's not a smear. For instance, stating that Clinton is corrupt, a habitual liar, a sexual predator, etc. is not a smear -- there are plenty of clear indications for all of the above.
I think that no public official should be exempt from smearing, including Ol' Beaver-Face Ashcroft. His opinion of opponents of the USSA-Patriot-in-Name-Only act probably makes Janet Reno hot.