http://itre.cis.upenn.edu/~myl/languagelog/archives/005294.htmlJosh's phrase dog whistle is itself a piece of political-operative jargon, discussed at some length in an earlier Language Log post ("The comma really was a dog whistle", 9/26/2006).
The first place that I ever saw this phrase was in a weblog post by Ian Welch ("Just a Comma: Dog Whistle Politics", The Agonist, 9/25/2006):The other name for this is dog whistle politics. When you blow a dog whistle humans can't hear it, but the dogs sure can. It's a pitch higher than humans can hear. When you speak in code like this, most of the time the only people who hear and understand what you just said are the intended group, who have an understanding of the world and a use of words that is not shared by the majority of the population. So it allows you to send out two messages at once - one pitched for the majority of Americans, the other pitched for a subgroup. This goes on all the time, and usually it isn't caught - most people don't hear it, and the media is made up of people who can't make the connections because they don't belong to these subgroups. So they can't point out the subtext either.
It's very effective, and it's one reason why Bush still has his hard core of support - he's constantly reassuring them, at a pitch the rest of us can't hear.
The "dog whistle" in that case was supposed to be President Bush's comment that the Iraq war is "just a comma". As discussed in the cited Language Log post, that particular argument turned out to be an extremely weak one, since the relevant use of comma was found mainly in the (left-wing) United Church of Christ, and was apparently unknown to the conservative evangelicals who were the alleged dog-whistle target.