First, if they are chosen to speak at a rally to introduce a candidate for president or vice president, chances are they're paying attention. These people are rabid political junkies.
Second, the campaigns brief speakers ahead of time as to what's acceptable.
Despite this, a campaign can't control everything.
Take Lehigh County Pennsylvania's Republican Party chair William Platt or Florida's Lee County Sheriff Mike Scott. Both men served as speakers to warm up the crowd before John McCain or Sarah Palin came on stage.
Although their remarks were quite different, there was a commonality between them that is causing the McCain team a lot of heartburn. Because that commonality takes away from anything meaningful that might be said at the event and focuses attention on their remarks.
(That is assuming that meaningful things are said at rallies.)
The commonality? The "H" word. As in Barack Obama's middle name. Hussein. It's something you just don't say. It's probably the second most disliked word in the entire McCain campaign (second to "poll").
But the mention of Barack's middle name is dicey. Usage could link the Democratic candidate to Islam (he's a Christian) or the reviled former dictator of Iraq.
Because of the implications, the "H" word is like touching the third rail.