In other words, state-level polls suggest that Biden has a national lead of around 8 points.
That's actually slightly more than the 6.6 points that Biden has in the national high-quality survey average taken over the same period. I should note that if we weight the average of the state polls to the population of each state, we get a margin just north of this mark of 6.6 points. (Weighting by population makes us a little more sensitive to outliers because we have fewer surveys from the most populous states.)
In any case, all methods agree that Biden has a fairly significant national advantage.
Examining state polls has the advantage of having a lot more data points to play with, so I feel pretty sure that they give us a decent snapshot. We are looking at over 20 surveys and over 15,000 interviews. The overall margin of error is small.
In addition, we can look at the states that we expect to be at least somewhat competitive (i.e. those where the margin was less than 10 points the last time) and those that we believe will not be close by. 2020.
In competitive states (where most of the polls were conducted), there was an average swing of 6 points towards Biden compared to Clinton's result in 2016. The same is true in non-competitive states.
At least from this state-level data, it does not appear that any of the candidates disproportionately scores in areas that were already favorable to them.
Biden posted tracks of more than 5 points in places like Michigan and Pennsylvania. It is ahead in more than enough states to win 270 electoral votes, if the election were held today.
We can also test our data to see what would happen if the polls underestimated Trump as they did in 2016.
What I found was that Biden would still be ahead, even with an accident the size of 2016.
Polls have underestimated Trump by 1 point (RealClearPolitics) or 2 points (FiveThirtyEight) in all of the states in which we currently have polls. By applying this 2016 bias to our current data, Biden would have a 6-7 point lead nationally.
By focusing only on competitive states, the polls underestimated Trump by 2 points (RealClearPolitics) or 3 points (FiveThirtyEight). If polls in competitive states were as low as at the end of 2016, Biden would still be ahead in states like Florida, Michigan and Pennsylvania.
Of course, it may not be wise to expect an election era the size of 2016 in 2020. Polls in those states that held major statewide contests in 2018 were pretty much impartial. Whatever group of states (all or just competitive) and what aggregate, the polls were no more favorable to the Republicans than the end result.
In a state like Wisconsin, the last Marquette poll of 2018 nailed the Senate's final margin and underestimated the Democratic candidate by the governor's margin by 1 point.
The bottom line is ahead of Biden at this time nationally and in competitive states. The good news for Trump is that he has about six months to change the course of the campaign, which is more than enough to do it.