As I wrote recently, the Speaker of the House, Paul Ryan (R), has raised the possibility that his party could lose its majority in November. This fear can account for a variety of recent political events, including the entry into the presidential race of a total unknown who just happens to be the policy director of the House Republican caucus.

To take control of the House, Democrats need to gain at least 30 seats over their current 188 seats. This is a tall order, but not impossible. A change of this size has happened in two out of the last five Congressional elections: a 31-seat gain for Democrats in 2006, and a 63-seat gain by Republicans in 2010. In the modern era of polarization, 1994-2014, a change of that size has happened in 3 out of 11 elections. Since 1946, it has happened in 10 out of 35 elections.

What would it take for the House to change hands?

One of the best ways to answer this question is not yet fully available. That best way is to look at district-by-district ratings by prognosticators such as David Wasserman of the Cook Political Report or Kyle Kondik at Sabato’s Crystal Ball, and ask what the average seat gain will be. I have written about this kind of information in the past (for example, it was dead-on in 2008). Those sites both currently give maximum plausible Democratic outcomes (i.e. certain/probable Democratic plus toss-ups) as 209 seats, short of the necessary 218 seats. However, the ratings take some time to be updated in response to events in the districts; after all, there are 435 of them, and it takes time for Wasserman/Kondik/their collaborators to collect and interpret incoming information. As accurate as this approach is, it gives a slow-moving picture that will surely change in the coming three months.

In the meantime, it is possible to get some feeling for what would have to happen at a national level. I will start off with an approximate question: what would the total popular vote for Congress need to be for Democrats to end up in control of 218 seats? And where is national sentiment relative to that threshold?

My tool for asking this quesiton is the general Congressional ballot. In the last six cycles, the measure at the end of October has come within 3 percentage points of the actual national vote. In addition, we have a second validation. As part of modern polarization, from 1992 to 2012 the generic Congressional has come within an average of 2.9% of the Presidential popular vote. The Clinton-over-Trump margin is currently Clinton +6.0+/-0.8% (median+/-estimated SEM, n=8) and the generic Congressional ballot is at Democrats +6%. So everything is aligned, up and down the ticket.

Now let’s take a look at the relationship between vote share and seat share since World War II: This relationship follows a broad relationship that is steeper than proportional. It contains many factors that you may have read about: incumbency and the relative packing of Democrats into cities, to name two. Those combined effects influence the location of the gray polygon, which circumscribes all elections from 1946 to 2010.

However, note that the most recent two elections in 2012 and 2014 fall outside the polygon. In these elections, which were unusually favorable to Republicans, something new happened: an organized wave of partisan gerrymandering. As I have analyzed in detail, the added advantage from post-2010 partisan redistricting amounts to about a dozen more Republican seats than what would occur under neutral districting principles. For those who are interested, the full story is detailed in a breezy but informative book, Ratf***ed by David Daley.

Yesterday, I used the generic Congressional preference poll and national Presidential polls to estimate that if the election were held today, House Democratic candidates would win the popular vote by 5-8%, indicated by the curly bracket in the graph. Judging from the last few cycles, that level of public opinion appears to be right on the edge of being enough to give Democrats control of the House.

Note that on the generic Congressional graph, I have displayed a threshold of Democrats +4.5% 7%. Revised – I may revise that threshold in the weeks ahead – informed comments are welcome.

Kyle Kondik takes a more extreme view, and estimates that Democrats need a +10% win. If true, that would be a serious deviation from 1946-2012 trends. There is a research finding from the 1990s asserting that the effects of gerrymandering can fade after a few cycles. Whether Kyle is right or I am, it is probably time to revisit that old finding – for instance with newer statistical tools.

So even though the general Congressional ballot is not as good as a district-by-district analysis, for now it is an up-to-the-minute tracker, and maybe the best measure we will get until October.