I honestly think Trumps greatest strength is that people REALLLLY underestimate him. He gets so much shit in the media, at work, in public, online, etc. and thus he seems way worse than he actually is in his supporters mind.
I think if people had just taken him seriously when he was running for president, and not laugh at so much of what he was saying, then he would have been crushed. Also, the media constantly attacking him only made him more popular.