Let us take a brief etymological tour, courtesy of the OED online, of “demagogue.”
It used to be a neutrally descriptive, if not an honorific, term, it seems; an example from the OED:
1719 Swift Let. to Young Clergyman, Demosthenes and Cicero, though each of them a leader (or as the Greeks called it, a demagogue) in a popular state, yet seem to differ.
Demagogue=pedagogue, in the sense that the former practices democracy as the latter practices teaching.
But in the very same era, the word was also used pejoratively, as it is now, and the neutrally descriptive connotation subsequently disappeared:
2. In bad sense: A leader of a popular faction, or of the mob; a political agitator who appeals to the passions and prejudices of the mob in order to obtain power or further his own interests; an unprincipled or factious popular orator.
a1716 R. South Serm. II. 333 (T.) A plausible, insignificant word, in the mouth of an expert demagogue, is a dangerous and a dreadful weapon.
One doesn’t hear or read the word very much in everyday media nowadays, perhaps because it sounds old-fashioned?
Another possibility is that the political spectacle of the pseudocracy is such that almost all politicians and many visible media beasts must be demagogues to do their jobs, as their jobs have become defined. Arguably, the main job of many state- and federal-level politicians is to be demagogues, and to govern is to practice a hobby, at best. If this is the case, then those interested in politics a) understand that politics = demagoguery now or b) think that a demagogue who agrees with them is a good leader (or candidate), and think a politician or candidate who disagrees with them is a demagogue–at best.
That is, one might posit that people with some critical distance from the whole process is likely to see almost all politicians as demagogues, albeit with differing degrees of demagoguery: neither Tom Coburn nor President Obama, for example, comes close to sinking to the level of
Rush Limbaugh or Donald Trump. One might then simultaneously posit that if the demagogue is being effective in an inappropriate, manipulative appeal to a voting bloc, then that voting bloc probably doesn’t view the demagogue as a demagogue.
Of course, political operatives like Karl Rove or James Carville will recognize a demagogue when they see one and applaud his/her behavior when the behavior serves a purpose with which they agree.