I don't think they will plant WMD for the simple reason that it's against human nature to "keep quiet." Is it possible? Sure, but I doubt it will happen.
Since you asked a hypothetical - I will answer hypothetically. If Bush's camp (I doubt he would directly order something of this magnitude) were to consider the planting of WMD it certainly would be a "last resort." SJ, surely you realize that until recently Bush had enormous and overwhelming support in the approval polls (despite Floyd and Patriot inaccurately insisting that polls are useless), so it would be inconceivable that any politician (mind you a president with a staff of masterminds) would risk something that could destroy their re-election bid to merely "increase support." If WMD were too be planted it would be only if Bush desperately needed a boost to contend (which is unlikely given the polarity of American politics) and relitively soon before the election (enough time to swing votes, but not to risk being exposed).
That is the only scenario that makes any sense, but like I said - it seems very unlikely that this could be pulled off and certainly the Bush camp must realize this.
Put it this way - as much as I distrust Bush, I would believe that discoved WMD were Saddam's until clearly proven otherwise.
And even if WMD were planted, it would be shock the hell out of me if Bush was proven to have been directly involved in the conspiracy.