Doing what they're doing right now but in swing districts is an obvious starting point, scare off black and brown voters from showing up at the polls. Especially in states with R controlled state governments that would go along with mail in voting restrictions if Trump demanded it.
Most of the competitive districts are mostly white and suburban/exurban/rural tho, and republicans gained with non-white voters so the possibility that they would be stopping their own is high. Not to even mention how that would motivate people who otherwise would not show up in a midterm to vote.
And there is not much overlap between competitive senate races and house races, there's just not enough capacity to take care of both, even if they were able to predict which ones would be the tipping point.