How much damage could a Republican Shadow Campaign forming NOW for the 2020 prez election? Should Trump start tweetingNOW about THAT?

The majority of American voters DISLIKE Trump and won't vote for him. His base isn't growing it is shrinking and will undoubtedly shrink even more as his supreme incompetence is revealed. Who will be left in 2020 to vote for him if he in fact survives that long?