So, first- you don't win an election with just your base in this country. Secondly, I'd be very careful about stats like that. You're assuming that political identification is a stable quality, and it's not. GOP party identification has fallen 3-5 points since 2016 (which is significant given that only about a quarter of people identify as Republicans in this country). So when you see polls that say "85% of Republicans support Trump!"- what that means is that most of the people who don't have stopped identifying as Republicans.
On top of that, independents are breaking away from Trump significantly. Right now, the only demographic that reliably supports Trump is the Republican-identified voting bloc. Support from white suburban women (who actually broke for Trump in 2016)
has dropped 8 points and appears to be in free fall right now.
Let's just call a spade a spade here: draconian voter suppression, extreme gerrymandering, and Russian state influence only go so far.
Finally, incumbent Presidential approval tends to drop once there's an actual contender in the race. When you're comparing someone to nothing, it's easier for them to get lukewarm approval. When there's another option, partisans solidify their support/disapproval, but independents tend to break away from the incumbent.