This is Part 2 in a short series projecting which party will win the House in 2020 given different possible presidential outcomes. Because it is a presidential election year and given the intensity of partisanship and increasing number of straight-ticket voters, the presidential results will be the best measure of the national environment and the most important factor in the House elections. This series will explore what those presidential results will mean for the House and which party can expect to win a majority of seats based on the various possibilities. You can find Part 1 here.

Quick Recap
If you did not read Part 1 of the article series, go read it for context. If you did (or are too lazy to go read it), here’s a quick recap. The goal is to determine what the 2020 presidential popular vote will need to be for:

  1. Republicans to be guaranteed to re-take the House
  2. Democrats to be guaranteed to hold the House
  3. Republicans to be likely (but not guaranteed) to re-take the House
  4. Democrats to be likely (but not guaranteed) to re-take the House
  5. Democrats and Republicans to be about equally as likely to control the House.

Last week’s post focused on #1 — what the presidential popular vote margin needs to be to create an environment where Republicans safely win the House. As you can see in the chart below, 4.7% is the end of “Safely Republican” territory and 4.6% is the start of the “Likely Republican” zone. Again, go read Part 1 to see exactly how this was determined.

Calculating the Likely Republican Presidential Popular Vote
Today we are continuing with the Republican side of the arch. What margin do Republicans need in the presidential popular vote to be likely to win the House?

Here are the variables we will take into account:

  • Each district’s 2018 margin. This will be the baseline for the 2020 results.
  • The incumbency advantage. FiveThirtyEight estimates that incumbents have a 2.7% incumbency advantage. So, for candidates who were incumbents before 2018, they will get no additional boost. Candidates who won an open seat, will get this 2.7% boost. Candidates who beat an incumbent in 2018 will get double the 2.7% (5.4%), to account for the incumbency advantage of their opponent that they overcame in that election.
  • Each district’s 2016 presidential vote: We will assume that the Republican loses in any seat in which Clinton won > 50%. These are highlighted in blue.
  • The 2018 midterm’s national environment. Because the environment leaned 7.3% in Democrats’ fav or, this will be subtracted from the Democratic candidate’s margin.

The chart below shows, according to the above variables, which Democratic seats Republicans are most likely to win. For this calculation, we will assume that Republicans win districts exactly in this order. Of course, this is not precisely how things would really play out, but it should, on the whole, represent what Republicans will need to win back these districts.

The districts in which Hillary Clinton won >50% of the vote, and which we therefore assume Democratic congressional candidates will win, are highlighted in blue.  

So, how many seats do Republicans need to win to fit our “Likely Republican” category. I am going to be mildly conservative in this calculation whereas for the “Safely Republican” category, I wanted a number that all but guaranteed a Republican victory, and was therefore extremely conservative. 

Here are the safeguards I am building into the calculation as to what Republicans need in order to “likely” win back the house. You can compare this to the “Safely Republican” safeguards, which were much more conservative. 

  • 5-seat cushion. Meaning I am estimating what Republicans need to win 223 seats, not the bare-majority 218.
  • Assuming that Republicans lose two of the four seats with candidates embroiled in scandal: Steve King (IA-04), Duncan Hunter (CA-50), Chris Collins (NY-27), Ross Spano (FL-15)
  • Assuming that Democrats win every seat where Hillary Clinton won >50% of the vote in 2016.

This means that Republicans will need to flip: 19 to take the majority, + 5 seat cushion + 2 to make up for seats lost due to scandal.  So, Republicans need to flip 19+5+2 = 26 seats.

The 26th seat that Democrats would flip is NY-19, which is north of New York City and encompasses much of the Hudson Valley and the Catskill Mountains.  This seat is currently held by Antonio Delgado, who beat an incumbent by 5.2%. If we adjust for incumbency (+5.4%) and the Democratic environment of 2018 (-7.3%), we find that, in a neutral environment, Delgado could expect to win his district by 3.3%. Therefore, Republican will need a 3.4% lead in the national environment, measured by the presidential popular vote, to likely win the House.

Now we can fill in the rest of the left-half of the graph. I have made a change to the graph to include a “Lean Republican” and “Lean Democrat” rating. This is the range where a party has a slight advantage, but one that could be easily overcome by just a few close districts swinging the wrong way. If Republicans win the presidential vote by in between 4.6% and 3.4%, they are likely, but not guaranteed, to take back the House. Since “Likely Republican” ends at 3.4%, we can fill in 3.3% as the start of the next zone, “Lean Republican”. The arrows indicate where new information was added.

The next post will complete the chart. We’ll calculate presidential popular vote for the Safely Democrat, Likely Democrat, Lean Democrat, Lean Republican and Toss Up categories.