Trump is probably on the list, so he has to protect that. The Democrats had it during their entire presidency; even if they didn’t want turmoil during their time in office, they could have released it after their presidential defeat and before Trump took office.
Simple, they have more to loose than what they stand to gain.
There are almost certainly several high profile democrats on the list, they don’t want to expose themselves so they are blocking the release.
But ok, let’s assume that the list has zero democrats on it, that doesn’t matter at this point, since it will be extremely difficult to find someone who the public can actually trust to give them real unbiased data.
The republicans knows that they can just lie and say that anyone in the democrats is on the list and just about everyone of their supporters will believe them
probably exposes alot of donors/megadonors to congress.