The fourth special election to the 116th U.S. Congress will take place on Tuesday, April 28, in Maryland’s 7th Congressional District. Kim Klacik (R) and Kweisi Mfume (D) are running in the election. The special election will fill the vacancy left by the death of Rep. Elijah Cummings (D) in Oct. 2019. Due to the coronavirus (COVID-19) pandemic, the election will be conducted by mail-in ballot only. Major independent observers rate the election as solid Democratic or safe Democratic.
As of April 2020, nine special elections have been called during the 116th Congress, the current meeting of the Senate and House of Representatives. Seven of those elections have been called to fill vacancies in the U.S. House. Three were held in 2019 (PA-12, NC-03, and NC-09), and four are upcoming (MD-07, CA-25, WI-07, and NY-27). Two special elections to complete terms in the U.S. Senate are scheduled on Nov. 3, in Arizona and Georgia.
From the 113th Congress (2013-2014) to the 115th Congress (2017-2018), 40 special elections were held: 16 in the 113th Congress, seven in the 114th Congress, and 17 in the 115th Congress.
During the 115th Congress, four of the 17 special elections resulted in a seat changing partisan hands, all flipping from Republican to Democratic. No partisan switches occurred in special elections to the 113th or 114th Congresses, and none have taken place in special elections to the 116th Congress.
There were 136 special elections between 1985 and 2012: 19 in the Senate and 117 in the House of Representatives. In those elections, nine seats changed partisan hands, with Republicans gaining six seats and Democrats gaining three.
Special elections to Congress occur when a legislator resigns, dies, or is removed from office. State laws governing vacancies determine when special elections are held.
https://ballotpedia.org/Special_elections_to_the_116th_United_States_Congress_(2019-2020)
Additional reading:
Special elections to the 115th United States Congress (2017-2018)
Special elections to the 114th United States Congress (2015-2016)
Special elections to the 113th United States Congress (2013-2014)