xenophobia.
UKIP and Trump are using the same kind of fear, anger, and nationalism that Hitler used. He promised to "Make Germany Strong" again and promised "pride", "work", and bashed the "elite" the Jews, and the Left.
Trump, and UKIP, are doing the same. There's a lot of America First sloganeering coupled with xenophobic scapegoats. The Muslims, the Mexicans are being painted as the underlying causes of Americas problems.
Hitler was considered a clown by the Left and the establishment parties. The industrialists who supported him thought they could control him and use him for their own purposes. The military thought the same and supported him because he promised to build up the military. They were wrong. Most of the German people ended up loving him because he was "Strong", "Got things done", "Decisive", "Patriotic", etc. They loved him because he offered simple (really simplistic) solutions to complex problems. He scorned the intellectuals as "eggheads" who did nothing and burned their books. He offered "realism". The Socialists are Communists are the problem. Get rid of them. The homosexuals, the artists, writers, cause weakness. Get rid of them. The Jews, Gypsies, and Slavs are the problem. Get rid of them. Not enough land? Steal it. Not enough labor? Make the conquered people slaves. And, on and on.
Trump like Hitler isn't smart, but he's clever like Hitler was. Play on the ordinary people's fears and anger and talk "patriotism", "success", "enemies", and "leadership". It's the "fuhrerprinzip" in blonde.
I never thought the American people would be stupid enough to elect Ronald Reagan "Morning in America" to any office. But, they did.