Couple of points:
1. “Demagogue” is pejorative. We use it to describe somebody who uses oratory to inflame passions and emotions in favour of some cause of which we disapprove. Thus we would call Adolf Hitler a demagogue, but we wouldn’t call Winston Churchill a demagogue, even though both men used emotional oratory to rally support for their respective agendas. In the 1930s, though, Churchill was regularly dismissed as a demagogue by his political opponents.
The result of this is that, when a politician is described as a “demagogue”, this tell us something not only about the politician but about the political stance of the person so describing him.
2. Oratory is involved. The second Bush Administration skilfully inflamed public fears over, and ignorance of, terrorism in order to marshal support for an unrelated invasion of Iraq of highly dubious morality. But even dear George’s warmest admirers wouldn’t regard him as an orator of any great ability, so we don’t consider him a demagogue.
3. My impression is that a demagogue uses his skills to inflame support for a cause, rather than simply to make us love him. A politician who just courts popularity, however oratorically skilful, is not a demagogue. He would, however, be a demagogue if he inflamed support for a particular cause in order to achieve power himself, or because he was genuinely and sincerely committed to that cause.
Bill Clinton, for example, is a skilful orator, and anyone I know who has ever met him, or attended a meeting at which he spoke, has fallen deeply in love with him, at least for at time. But I don’t think that’s enough to make him a demagogue. To become a demagogue he would have to do more than say, in effect, “elect me as President and I will do the best job I can”; he would have to be identified with, and courting support for, a specific programme.
In this sense, US politics doesn’t often produce demagogues, in that elections are mainly about selecting the most appealing candidate, the candidate whose character, judgment and values you trust. Elections are not so much about choosing between fundamentally different political programmes, and candidates rarely win by advocating, e.g., radical monetary reform, or war with Mexico; they win by getting people to trust them. William Jennings Bryan was a true demagogue, I think, his twin causes being bimetallism and racism, but John F Kennedy, not so much, since his oratory was mainly directed at making people feel (a) good about him, and (b) good about themselves.
Is Obama a demagogue? Yes, he’s a good orator and, yes, he gets people to like him, even to admire him passionately and irrationally. But I don’t think that’s quite enough. Perhaps I’m not close enough to US politics, but I don’t see him as whipping up enthusiasm for a particular programme of sufficient significance, unless you count healthcare reform. If you do, and if you oppose his healthcare reforms, then you could argue that he is a demagogue.