Now that all the gnashing of teeth has ended after the Republicans managed to hold on to the Georgia Sixth, perhaps we can step back and take a more systematic look at the Democrats’ prospects in 2018. Democrats will likely not make any gains in the Senate since the Republicans have only eight seats at-risk compared to twenty-three Democrats and both independents, Maine’s Angus King and Vermont’s Bernie Sanders. That leaves the House as the only target.
There are two steps involved in answering this question. The first is to use our historical experience with House elections to examine how votes are translated into seats. With that information we can estimate the proportion of the two-party House vote that the Democrats need to win to take back the House in 2018.
As I wrote back in 2012, a combination of geographic clustering by party and good old partisan gerrymandering has created a “Republican bulwark” in the House since the last redistricting after the 2010 Census. That means that the Democrats will need to win more than a majority of the popular vote for Congress if they intend to win a majority of House seats.
I have refined this simple seats and votes model in two ways. First, I let the “swing ratio” vary between two historical periods, 1940-1992 and 1994-2016. Empirically the effects of voting “swings” on seat “swings” is significantly smaller in the more recent period. As Tufte argues in his classic paper on the seats/votes relationship, a decline in the swing ratio indicates an increase in the proportion of “safe” seats. As fewer and fewer seats have vote shares around fifty percent, there are consequently fewer that can be “flipped” by an equivalent shift in voters’ preferences.
I also use the results for the 2014 and 2016 elections to more sharply estimate the effect since 2010. If we calculate the popular vote share required for the Democrats to win half the seats in the House, they would need to secure a bit over 53 percent of the (two-party) votes cast.
That brings us to the second question, what are the chances that the Democrats could win 53 percent of the Congressional vote in 2018? Answering that question deserves an article unto itself.
