There have been nine congressional (House and Senate) and governor elections with polling in 2017 and 2018, and the polling from them has been pretty good, historically speaking. Let's take a look first at the six special House elections with polling in the final three weeks of the campaigns: Arizona 8, Georgia 6 (the special election and runoff), Montana at large, Pennsylvania 18 and South Carolina 5.
That's why it's not surprising that the polls have not been biased against the Republicans in the off-year cycle. Across the eight races this cycle with polls in which there was only one Democrat and only one Republican, the Democrats have done on average 1.5 percentage points better than the average poll in the final three weeks of the campaigns.
The errors have been mostly random. Republicans did better in the Georgia 6 runoff and Pennsylvania 18 special. Democrats did better in the Alabama Senate and Virginia governor's races. The randomness of the errors is exactly what you'd expect given history.
None of this is to