They really won't have to do much.
Americans will suddenly remember how much they hate Trump and have buyer's remorse over letting him return to power. Just like 2018, but on steroids.
It'll be a bloodbath for the GOP, but a pyrrhic victory for Democrats at best since Trump would still be President and probably already did irreparable harm in his first two years.
What tanks Trump's popularity in his second term?
Literally just being Trump all over the news again.
Nope. That's become normalized. He's already saying unhinged things every day on the campaign trail. People just accept it and a lot of people like it.
Democrats can't just run on "not being Trump"
Midterms are a referendum on the incumbent, and Trump will not be able to fall into the background like Biden did.