On a daily basis, Morning Consult surveys registered voters across all 50 states on their elected officials, voting preferences, and more. The results for the 2018 Midterm Wave Watcher were generated by compiling those daily surveys into three month rolling averages for each data set. For example, the most recent data set (May) comes from the results of surveys taken between March 1, 2018 and May 30, 2018.
The data were weighted to approximate a target sample of registered voters based on age, race/ethnicity, gender, educational attainment, and region. Respondents in each state answered questions specific to their senators.
The margin of error for the national averages are +/- 1%. At the state level, the margin of error varies from +/- 1% to +/- 6%. The majority of states have a margin of error below 2%.
Below are the time frames and sample sizes for each data set:
-January: Surveys with 256,590 registered voters from November 1, 2017 to January 31, 2018
-February: Surveys with 263,374 registered voters from December 1, 2017 to February 28, 2018
-March: Surveys with 275,377 registered voters from January 1, 2018 to March 31, 2018
-April: Surveys with registered 276,786 voters from February 1, 2018 to April 30, 2018
-May: Surveys with 314,155 registered voters from March 1, 2018 to May 30, 2018