First, is the current polling seems nuts and I don't think it is accurate. 

Second, name recognition is huge in politics and nobody keep their name out in the public forum like Trump. The guy learned that all publicity is good, doesn't matter if you are being charged with crimes or quoting Hitler, as long as the media talks about you, it is a win. He has the most recognizable name in politics. 

Third, the Republicans have been using the same playbook since the Hillary Clinton arrived. Which is smear your opponent, label them a criminal, never provide proof, just keep repeating they are a criminal, baby killer, communist, far left lunatic. Build as much hate for them as you possibly can. This method is tried and true with Republican voters. 

Fourth, Trump has stirred into this volatile mix, pretty much never tell the truth, blame others for anything that goes wrong, and as soon as someone points out your behavior, claim the opponent is doing it. 

Trump also knows that negative works better than positive in politics so he stays in negative attack mode at all times. Against everyone, so he just tells people "I'm the greatest ever! Everybody else stinks!" and sadly that works too. 

Because people are so crazy misinformed they have no clue how giant his lies are. 

His rallies are this weird combination of stand-up comedy and list of grievances covered in fascism gravy. He's now told so many lies that there is no way for him to tell anybody the truth now. Lying is his brand.

All this worked for him basically for two reasons, one that there is a willing audience for the kind of hate he sells, and two he beat Hillary Clinton because she was the most divisive politician in American politics until he arrived. He's now taken that away from her as well. 

I honestly don't see any evidence he is gaining popularity, he seems to just be driving more and more people away, and so, it looks like, no matter what current polls say, he has easily lost the people that got him elected in 2016...which were independents and people that voted for Obama. So, having lost those people, he does not seem in a position to win an election. He's not the "outsider" he was in 2016, he single term as president was a total disaster, and when he left office the country, for the final two years of his presidency was a catastrophe. 

I certainly don't understand the people that still like him, but I honestly believe most of them, primarily do not really know about anybody else except for Trump. I think we'll have to see what happens when Vivek and Christie drop out and it is just Haley and the Donald. Perhaps then his polling numbers will erode.