The Republican Party holds 51 seats in the Senate. Democrats hold 47 seats, and the remaining two are held by independents who caucus with the Democratic Party. The Democratic Party faces greater partisan risk in 2018, as they are defending 25 seats (two of which are held by independents), while eight seats up for election in 2018 are held by Republican incumbents. The Democratic Party must defend seats in 10 states that supported Donald Trump (R) over Hillary Clinton (D) in 2016.
Republicans also control the U.S. House of Representatives. As of September 2018, the Republican Party was in the majority, holding 236 seats to Democrats’ 193 seats, with six seats being vacant. The Democratic Party is well-positioned to gain seats in the chamber in 2018; since 1934, the party of a newly elected president has suffered an average loss of 23 seats in the House in the following midterm.
In a collaboration with political scientist Jacob Smith, Ballotpedia analyzed election data from 1918 to 2016 and defined wave elections as the 20 percent of elections in that period where the president’s party lost the most seats. For the U.S. House and the U.S. Senate, this means that Republicans need to lose 48 House seats and seven Senate seats for 2018 to qualify historically as a wave election. Read the full report here.
Ballotpedia has compiled the following resources to help voters better understand the landscape and consequences of the midterm elections in 2018: