He's referring to the aggregation of votes for Democrats and Republicans nationally, as in, more people nationally voted for Democrats than for Republicans.
It's a meaningless figure (not least for the Senate, because the Senate race in the largest state was between two Democrats, with no Republican to get any votes).
How is it the "worst in 100 years"? The exact same states are up every six years (barring any special elections). These were the races up for grabs, pre-election, 2012:
There were races, pre-election, 2018:
They're almost exactly the same map, except it's a little
worse for Republicans.
I said they did well, but they didn't do
unusually well.
Look, if you have to handicap races to "explain" why the Democrats didn't do
better, then you must not think they did superbly well either.