I still think a bigger part and the core part though is he speaks specifically what the majority of the working class think. The working class doesn't want open borders, they don't want illegal aliens, they don't want mass amnesty while maybe arguments can be made depending which side for limited very small amnesty, they don't want foreigners getting taxpayer benefits, and they do NOT think Trade Deals help them at all. The average working class American is not as dumb as the politicians act while many are uninformed on issues the Media tries to bury or ignore. No one thinks trade deals with any other country be it Mexico, South Korea, or wherever else benefit the working class or middle class. No one really thinks illegal aliens do jobs Americans won't do. And no one thinks foreign legal workers doing a regular non super skilled job benefits the average American at all.

So basically they like his message. Secure the border, get rid of illegal aliens. and bring back American jobs lost due to bad trade deals and policies.