Methodology for The Nation article, Its Safe to Impeach Donald Trump, published on 8/13/2019.
Number of Trump-won districts = 21
Remove districts where Dem 2018 vote was less than 2016 Clinton vote = 7
A. Those were won with just turnout
B. 14 Districts where Dem 2018 vote greater than 2016 Clinton vote
Remove districts where if you took away all of the Dem increase and gave it back to the Republicans, the Dem would have still won
A. Did not need defections; HRC vote would have been enough
B. Dem margin of victory was at least 2x the size of the increase in Dem vote
C. Remove 6 districts from 14, leaving 8 districts
Of those 8, there are three special situations with other, more likely, explanations for why the district flipped
GA-06: The number of Democratic votes increase by 5,062 votes, and Lucy McBath won by just 3,264 votes, meaning if you took all of the increase above Clinton’s vote number and gave it to the Republican Congressional candidate, McBath would have lost. But, 2018 saw Stacey Abrams’ campaign galvanize the most historic increase in Democratic votes in the history of Georgia, making it at least as likely that McBath’s increased vote total came from new voters who sat out 2016 than from defections of disillusioned Republicans
NM-02: The number of Democratic votes increased by 8,126 votes, and Xochitl Torres Small won by just 3,722 votes. Here, there likely were a number of people who voted for Trump in 2016, but a dominant feature of this district is that it is 55% Latino. For 15 years the seat was represented by a relatively popular white Republican, Steve Pearce, but Pearce didn’t run for re-election in 2018, and that presented the first time that the seat was both open and had a Latino Democratic nominee. The facts that there no longer was a popular incumbent and that there was finally a strong Latino candidate were likely as significant as disaffection with Trump in determining this race’s outcome.
UT-04: Here there was a huge increase in Democratic votes (45,168) and a teeny-tiny margin of victory (just 694 votes), meaning that there almost certainly vote-switching swayed the outcome. The reason for the switching in this district, however, likely had much less to do with defections from Trump than with the long-standing, ugly realities of race and gender in American politics. Republican Mia Love was the only Black woman Republican in Congress, and she rocketed to fame in 2012 as the poster-child of a Black person opposed to Obama. That went over quite well, and she came so close that she rode that fame and support to a rematch in 2014 where she prevailed. By 2018, however, being the token Black person against Obama mattered less, and politics reverted back closer to the mean. Absent the unusual circumstances of Love’s rise, a well-known, popular former mayor, white male Mormon defeating a Black woman in Utah is pretty much what you would normally expect to happen, based on decades of electoral and demographic data.
And that leaves just 5 Congressional districts where disaffected Republicans were likely the main determinant: