Pre Kavanaugh, the Dems were almost certain to win the house... post no chance in hell.
The Dems actions did something you can’t ever do in US politics...
Anger the middle/casual voter.
Polls are not even picking up probably 1-3% of the voters who will show up and overwhelmingly punish Dems, because they are filtered out as “unlikely voters”, because they are not regular voters. They are only showing up this time to let the Dems know they went too far.
Kavanaugh hearings and the mob mentality turn normal people off.