You never hear of a Republican overtaking a slim Democrat lead in the final stages of a “close” election. It’s always the other way around. And usually after a box of uncounted ballots mysteriously turns up in the trunk of somebody’s car.
It would be very interesting to see an empirical study analyzing the number of tight races that went to Democrats over the past few decades. The data is readily available and would probably show that something is persistently tipping the scales and/or reversing initial voting results.
The RNC should be all over this. But they’re never going to do it.