I would do two things, both seemingly disconnected:
First, I would run ads discrediting Trump, and exposing Trump's deplorable behavior. I would flood social media, but specifically targeting people who are more inclined to see the truth. I would spam every social media platform with this stuff. I would use bots to spread it everywhere.
But - and this is important - the story would be subtle. Bread crumbs dropped that lead, over time, to the inescapable conclusion that Trump is a fascist.
You don't just jump right out there and call him a fascist. You start by telling a story about how things used to be, and how we can get back there again. You move to a story about how Trump has done things to move us backward while pretending to move us forward, exposing his lies. You then tell a story about the consequences of his current polices and his lies. Then, finally, you show how this path leads to an authoritarian government, with Trump at the head.
Second, and in parallel to the above, you tell a story about how the democratic candidate has done things to solve these problems within his sphere of influence, and how those changes could be expanded on a national level given the opportunity. You tell a story about the path to freedom and the preservation of our Constitution and Republic. You tell a story about the creation of jobs and increased opportunity for everyone.
Understand that this is all done with slightly different messages to slightly different groups. You tell each group what they need to hear to vote democrat. You do this constantly. You stay in their line of sight all the way through the election. But you do it without them knowing that you are doing it. You make it appear as if it is coming to them from organic sources, not the candidates themselves. You make it look like all of this is coming from their friends, or sources that they trust.
That is how you win an election in 2020.