Should Democrats win big in 2020, then such will reflect the gross failure of the Republican Party. I cannot tell you yet what the symptoms will be. There could be an economic meltdown that discredits the economic stewardship of the Republican Party. Republicans now own the economy and can't blame Democrats for any failure. There could be a military or diplomatic debacle with the military and the Trump or Pence Administration pointing fingers at each other. There could be civil unrest as the government shows callous disregard for a majority of the population.
The ideal subject of totalitarian rule is not the convinced Nazi or the dedicated Communist but instead the people for whom the distinction between fact and fiction, true and false, no longer exists -- Hannah Arendt.