It's not so much that Democrats have been winning; it's by how much they have been winning. For example, in a special election for a Virginia Congressional seat, the winner, James Walkinshaw, won by a 50-point margin. By comparison, Kamala Harris carried the same district by 34 points. Similarly, last April in two special Congressional elections in Florida, the Democratic candidates overperformed Harris by 16 and 23 points (but not enough to win the open seats).