Ballotpedia has identified 79 U.S. House battleground races: 70 Republican seats and nine Democratic seats. Heading into the elections, Republicans have a 235-193 majority with seven vacancies. To win a majority, Democrats need to have a net gain of 23 Republican seats.
The Democratic Party is well-positioned to gain seats, according to a 100-year historical analysis of House elections conducted by Ballotpedia and political scientist Jacob Smith. From 1918 to 2016, the president’s party lost an average of 29 seats in midterm elections. In the 20 percent of elections where the president lost the most seats—which Ballotpedia defined as wave elections—his party lost at least 48 seats.
The party of a newly elected president gained seats in the House in the following midterm only twice. Democrats gained nine seats in 1934 following Franklin D. Roosevelt’s (D) first presidential election in 1932, and Republicans gained eight seats in 2002 following George W. Bush’s (R) election to the presidency in 2000.
Source https://ballotpedia.org/U.S._House_battlegrounds,_2018
