Trump has a really, really, really dedicated fanbase. That doesn't translate to mainstream appeal, hence why his approval ratings struggle to get out of the low 40s and his disapproval ratings continue to average above 50 percent. It's kind of like how Touhou games have a very dedicated fanbase but no real mainstream appeal.
Trump has a rabid fanbase because his entire presidency has been based on throwing out red meat for said base. He has absolutely no filter and says the first thing that pops in his head with no concern of the consequences. With the GOP, for the last 40 years or so it's always been about "God, guns, and gays," and now also immigrants. It's a fear-based platform: fear of "foreign invaders," fear of communist takeovers, and fear of anything that conflicts with fundamentalist Christian values. The GOP base, at their core, is ruled by gut feelings and fear of "the other," not by a desire to formulate rational policy decisions regarding serious issues that affect all Americans on a daily basis. These attitudes are nothing new, either, and have antecedents going back to the Jim Crow era, the Know Nothings, and other sorts of eras and movements that are a stain on American history.
For a long time, though, Republicans had to use dog whistles in an attempt to avoid saying anything too outrageous. But as conservatives forced the Overton window of American politics ever further to the right, Trump and the talking heads on Fox News and right-wing radio have simply traded the dog whistle for the bullhorn. Whatever decorum was left in Washington, Trump burned it all down. That excites the base, because they see it as brave. They like seeing someone repeat what they've always said in private company, and without the coded language. Trump can say anything about immigrants, liberals, and anything else his base hates, and no matter how vile he gets his base will applaud him. Trump can implicitly support violence against his opponents and critics and his base will applaud. Trump can threaten to imprison his political opponents and shut down journalists critical of him, and his base will applaud. They do that because they think anyone who doesn't agree with them aren't just wrong, but dangerous. They think "the left" (insofar as much as America really has a proper left-wing) are literal Stalinists that are intentionally out to undermine everything good and decent in America and thus must be stopped dead in their tracks, no debate, no compromise. Of course, their view of a "good and decent society" is one structured just like it was circa 1950 or earlier, where "certain people knew their place," where "Bolshevism" was kept at bay, and where God-fearing white men could openly say and do whatever they wanted to those beneath their station. These are people ruled by base impulses and reactionary attitudes, and now they have a President who's just like them. I know, because I've known many of them my whole life. I hear what they say in private, things that are no longer safe to say in public without getting the stink-eye from someone. When I was a young(er) man, what I heard from family members and friends of the family disgusted me, and I'm glad I had enough sense not to fall into that worldview. My conservative family inadvertently made me the progressive I am today.
Trump's rhetoric may play well with the base, but while keeping the base happy is important it is not the sole determinant in being a successful presidential candidate. Keeping at least a plurality of the middle is necessary, too, and in that regard Trump has failed spectacularly. Just because saying something outrageous gets you brownie points with your base doesn't mean it'll fly over well with the rest of the country, and that's important because self-identified Republicans represent at most a third of all voters (though to be fair self-identified Democrats make up about a third of voters as well). Trump has struggled to appeal to the middle with his deliberately divisive rhetoric, hence his poor approval ratings. Hell, he didn't so much win in 2016 as Hillary lost, as he won by razor-thin margins in several swing states, he didn't do any better than Romney did in 2012, and he won with the smallest popular vote share of any major-party candidate in an election without a major third-party/independent contender (defined as earning over 10% of the popular vote and/or winning at least one state). Then-candidate Trump's promises to bring back manufacturing and revitalizing the Rust Belt may have won him some votes in 2016 as well, but after 2-½ years it's clear that was largely empty promises, and looking at the results of the 2018 midterms as well as state-level approval rating data the Midwest appears to be having a blue-shift back to where it was before.
With Democrats, well, the last 18 years have made it abundantly clear that they cannot drum up the same level of excitement in their base. It's not that progressives can't get excited like conservatives can, but they are much harder to excite. There is a contingent of progressive Democratic voters that simply aren't satisfied by anyone else other than a "true progressive," and they rarely get that as the mainstream Democratic Party is a largely centrist party. You occasionally get someone like Bernie Sanders (who isn't a Democrat, even if he does run under their banner in his presidential campaigns) that gets progressives excited, but the Democrats' rhetoric by and large isn't a progressive mirror image of modern right-wing rhetoric. They generally don't make a habit of saying outrageous things to keep the hardcore base happy. We're far more likely to just hear more "The rich should pay their fair share" from Democratic candidates than we ever will hear something like "Corporations are the enemy of the people" or "Lock up Mitch McConnell." Even Bernie Sanders is very level-headed and policy-focused in his speech, the polar opposite of Trump. This a party predicated largely on policy and building a broad coalition, not simply throwing red meat to the base. They are by and large simply too moderate and straight-laced as a party in their general attitude to whip their base up into a frenzy. And unlike conservatives, when progressive don't get the candidate they want they sit at home on Election Day or vote third-party out of protest. "Bernie or Bust" types had direct parallels to progressives in 2000 who wanted Bill Bradley as the Democratic nominee instead of Al Gore, and when they didn't get that they either sat at home or voted for Ralph Nader. Progressives are very, very hard to satisfy.
In this current election cycle, the fact that we have over 20 candidates also means that the Democratic base is split all over the place, with no clear favorite for the majority of them to coalesce around. As the playing field gets whittled down and primary season begins early next year, I hope we start to see more excitement out of the base. Whether it's Biden or Sanders or Warren or someone else, I hope they can use the simple prospect of defeating Trump to excite a notoriously hard-to-excite base. Otherwise we'll have the opposite problem Trump has: appealing to most of the middle, but failing to generate enough turnout in the base. If the base turns out in 2020 even if they don't personally like the candidate, the odds will be in the Democrats' favor. Trump is unpopular, and presidents this unpopular have never won re-election, but there's a first time for everything. The Democrats have to play this smart, and their base needs to show up. No more of this "Bernie or Bust! No corporate Democrats!" garbage. I don't care for Biden, either, but I'll vote for him because I want Trump out of office.