They don't need to maintain control. Elections are close and voters are geographically polarized.

Reduce turnout in cities in four or five swing states and that can be enough to flip an election. If you've got ICE just detaining random non-white people near polling places based on the theory put forward in Noem v Vasquez Perdomo, how many nonwhite citizens who would be solid voters for the democrats just decide to stay at home rather than go to the polls?

It is possible that this approach sparks a reaction and backfires. But I'm not sure that's a guarantee.