Wave watch: How the special elections could predict the midterms

Notes: Pennsylvania's Congressional districts were redrawn by the state's supreme court in March 2018. This map reflects the updated boundaries and adjusted PVI. Data: Cook Political Report Partisan Voter Index, Ballotpedia; Map: Chris Canipe/Axios

Democrats have overperformed by an average of nine points in each of the eight special elections since President Trump took office — giving us a sense of what a blue wave might look like in November. This map shows the scenarios for the House races if the Democratic voter turnout resembles the special elections.

Yes, but: Past performance doesn't guarantee future outcomes. A 9-point swing would put the 2018 House race on par with Republican waves in 1994 and 2010, but the generic ballot isn't as favorable. Real Clear Politics had the Democrats up by 13 in December 2017, but by end of this month, their lead was down to 4 points.

How to read this map: The Cook Partisan Voter Index (PVI) estimates the partisan leanings of a district by comparing the average Democratic and Republican share of the presidential vote in the past two elections to the nation as a whole.

  • For example, D+5 means the Democratic candidate performed an average of 5 points better in that district than in the nation as a whole in 2012 and 2016.
  • Likewise, R+5 means the Republican candidate performed 5 points better than the national average. Districts with a lean of less than half a point are ranked "even."

Democrats won only two of the special elections since Trump took office, but they've overperformed in all of them. The margins:

  • Kansas 4th, April 11, 2017: James Thompson (D) lost to Ron Estes (R) with 47 percent of the vote, for a total swing of 12 points.
  • Montana at-large, May 25, 2017: Rob Quist (D) lost to Greg Gianforte (R) with 47 percent, for a total swing of 8 points.
  • Georgia 6th, June 20, 2017: Jon Ossoff (D) lost to Karen Handel (R) with 48 percent, for a swing of 6 points.
  • South Carolina 5th, June 20, 2017: Archie Parnell (D) lost to Ralph Norman (R) with 48 percent, for a swing of 7 points.
  • Utah 3rd, Nov. 7, 2017: Kathie Allen (D) lost to John Curtis (R) with 31 percent, for a swing of 6 points.
  • Alabama Senate race, Dec. 12, 2017: Doug Jones (D) beat Roy Moore (R) with 51 percent, for a swing of 15 points.
  • Pennsylvania 18th, March 13, 2018: Conor Lamb (D) beat Rick Saccone (R) with 50 percent, for a swing of 11 points.
  • Arizona 8th, April 4, 2018: Hiral Tipirneni (D) lost to Debbie Lesko (R) with 47 percent, for a swing of 10 points.

Go deeper: The open House seats in the 2018 midterm elections

Correction: This map has been updated to correct a data transcription error that caused the partisan lean for Pennsylvania's newly redrawn districts to be associated with the old district numbers.