From The Collaborative International Dictionary of English v.0.48 [gcide]:
Demagogue \Dem"a*gogue\ (?; 115), noun [Gr. dhmagwgo's a popular leader; commonly in a bad sense, a leader of the mob; dh^mos the people + 'agwgo's leading, fr. 'a'gein to lead; akin to E. act: cf. F. d['e]magogue.] A leader of the rabble; one who attempts to control the multitude by specious or deceitful arts; an unprincipled and factious mob orator or political leader.
From WordNet (r) 3.0 (2006) [wn]:
This man is DANGEROUS. He is a pathological LIAR. He is INDIFFERENT to the TRUTH, FACTS, EVIDENCE and PROOF. Those are the cornerstones of JUSTICE. Those are the FIRST PRINCIPLES of SCIENCE. This is a glaring EXAMPLE of a NARCISSISTIC SELF-AGGRANDIZING EXAGGERATION. This is two blatant LIES in 18 seconds. The first LIE: "Nobody KNOWS the SYSTEM BETTER than me." That is a blatant LIE. There are many EXPERTS who KNOW the SYSTEM much BETTER than Donald Trump. Then he says, "Which is why I alone CAN fix it." That's NOT TRUE. Most likely he CANNOT fix it. There are others who CAN. That's two blatant LIES in 18 seconds.