Republicans outperformed expectations in Congressional results. This is unusual given congressional results and presidential results have historically correlated. I don't understand the explanation for that if Democrats rigged the election somehow.

It seems more plausible that:

- there was a really high turnout aided by widespread absentee voting

- a large single-digit percentage of voters went Republican in the congressional races but didn't vote for Trump

I can't think of a fraud based scenario with any factual support that is as compelling.