I am using RCP as a pretty reliable source for this material (though they sometimes conviently forget a poll that isn't conservative enough for them). For this thread to work over a long time, you simply need these 2 links: 2020 Congressional ballot 2018 Congressional ballot So, let's get started. Here are the 2020 polls from the middle of August 2019 until now (a span of two months): And here, the same time-span in 2017 for the 2018 generic ballot: I'll be updating this about once a month. -Stat
Time for an update. When I created this thread on October 16th, 2019, the aggregate was D +7. Today, the aggregate is: D +7.7
One useful tidbit for this thread right now is the primary dates, noting those that coincide with the Presidential preference primaries. A few of those (Alabama for example) were the same day in 2016 and impacted both primaries greatly.
At this point in time in January of 2018, the RCP aggregate had Democrats up by 11.4% and the RCP average ended with Democrats up 7.3% (a drop of 4.1%). Democrats ended up winning by 8.4%. 538 also tracks the generic balloting data at at this point in time in January of 2018, the 538 aggregate had Democrats up by 9.9% and the 548 average ended with Democrats up by 8.7% (a drop of 1.2%). Again, Democrats ended up winning by 8.4%. As an initial point, I find it interesting that in comparison to each other, RCP started as more favorable to the Democrats, but ended up with a slightly more pessimistic view, when compared to 538. The ending can probably be boiled down to the fact that RCP incorporated the final (and absurd) Rasmussen poll that suggested Republicans would win by +1 while 538 appears to have ignored that poll altogether. In terms of the impact of the data on 2020, 538 currently has Democrats with a lead of 6.6% and RCP currently has Democrats with a lead of 7.2%. Given the historical data, I would predict that the generic balloting to tighten a little further - perhaps to somewhere around Democrats by 5.5%-6.2%.
As an update to this data, now that we are nearly two months further along, 538 has Democrats with a 7.3% lead and RCP has Democrats has a lead of 9.0%. That is a 0.7% and 1.8% increase from the last update, respectively. At this point in 2018, 538 had Democrats with a lead of 8.3%. RCP had Democrats with a lead of 7.8%. Again, Democrats ended up winning by 8.4%. The data from these generic polling appear largely accurate even this far out.