It's complicated but basically it all goes back to his stance on immigration. The democrats' master plan is to overwhelm white America with Hispanic immigration and turn the whole country into California. Trump came out foursquare against that and they know that he alone threatens their plan. The establishment republicans have sold out to business interests who want unchecked immigration to get low wage workers and for population growth.
The problem is that Trump's actual plans, not only on immigration but on making european and Asian allies pay for their defense, for a halt to muslim immigration and against trade deals, are pretty popular with voters, so the media and democrats cannot just attack his policies. Instead they have created a caricature of Trump as an unhinged madman who cannot possibly be let near the levers of power. They have been aided by nominal republicans who are in the pocket of the defense industries or businesses that depend heavily on importing from china and asia.
