Offer them something better. They support Trump because he makes them feel:

1. Smart for cutting through all the lies to get to the truth.
2. Part of something bigger than themselves.
3. Victimized/marginalized (few things are more empowering than being wronged).

The opposition has to overcome these powerful emotions. Currently, they are kinda banking on a decisive victory in court. That might be enough to dispel some of the enchanted, but will only encourage the most fervent. Worse, if it doesn't get him locked up or disqualify him from running, it's going to increase his power.

The best way to fight it is with a candidate that can beat him. I hope Biden is capable of pulling it off a second time.

Besides Trump himself, there are systemic issues that we should be fighting over. The electoral collage system is a huge problem that isn't going away. 2000 and 2016 elections won by candidates with fewer votes.

Gerrymandering is also a big reason why he has as much support as he enjoys. Honestly, I might not care as much if he were president if Congress and State legislatures weren't so out of wack with our population distributions. Representative government my ass. Land is straight up voting and it should piss us all off way more.