I've noticed something about presidential elections.
Charisma above all. Almost all presidents since I've been alive - and some beforehand - have checked this box. The only one that didn't come off as charismatic IMO was HW. Think about it:
Trump: Hugely polarizing, but clearly more charismatic to the voting public than Clinton. He was greatly aided by an outsider, populist image in a greatly anti-politician/anti-establishment era.
Obama: Oozed charisma. Nuff said.
W: I've heard lots of people describe him as "a guy I'd like to have a beer with." He was aided by Gore coming off as bookish and somewhat elitist during debates and of course, Kerry got swiftboated.
Clinton: First black president. Played his sax on national TV. Young, fresh politician compared to four more years of HW. He got impeached for sexual impropriety, and his approval ratings went up.
HW: Doesn't really check this box. Rode Reagan's coattails into office and the fact he lost to Clinton helps bolster the argument he wasn't necessarily charismatic enough to win.
Reagan: Hugely popular. Still worshiped in some circles as the pinnacle of Conservative America.
Carter: Still a likeable guy and represented a clean, ethical start after the final term of Nixon. Ford was also kind of a bore, which helped.
That's all the further I care to take back this hypothesis. But you see my point. Correct me if any of these points are wrong... I'm operating on secondhand knowledge and my own impressions for a lot of these. In the cases where charisma was questionable, winners were aided by a weak opponent in that regard.
As for Trump... In terms of pure partisans: The GOP base is clinging tightly. Democrats will never go for him in any meaningful numbers.
I just saw a story today saying his approval rating among Independents is steadily declining since January. He started out just above water with them overall, and he's down to -21 with them. He is hovering consistently right around net 0 approval for his handling on the economy, however. They used YouGov polling numbers, which get a solid B on 538's rankings.
Here is the article if you're interested.
Other polls I've seen seem to have Indies having about a 1:2 approval-disapproval numbers on a lot of topics. Overall, he seems to be losing ground there.
As it pertains to this discussion, as Red pointed out, Indies or swing voters are key. Base turnout will be there, and candidates will need to juice it. But you win elections on persuadable voters that could go either way. Insofar as Obama-Trump voters, they both voted for a change candidate who made changing Washington a central part of their campaign. Given the right track-wrong track numbers, change messages generally sell very well at the presidential level.
IF Trump's ratings continue to trend the way they are, he'll be pretty dang unpopular in 2020. As far as I'm concerned, if the Dems run a charismatic candidate with a change message (youth could help in this regard), they've got a very good shot at knocking him off. A young candidate could compare favorably with the septuagenarian Trump. In addition, usurping the change message from Trump and turning him into the status quo candidate would be powerful.
The Democrats problems are more systemic and prevalent at the state/Congressional level, IMO.