When congressional elections are highly contested, it is usually because there is an open seat. An open seat is a political office whose incumbent resigns, dies, or chooses not to seek reelection. In such situations, both parties will strive to win that seat, though one party might not mount a strong challenge for a seat that the other party has a history of winning by a large majority. The number of open seats in any given election year is usually a small proportion of total congressional seats.
Redistricting and Gerrymandering
How congressional district maps are drawn contributes to the lack of competitiveness in House elections. The boundaries of congressional districts are redrawn every 10 years, after each national census of the population. Some states gain or lose seats in Congress because of population changes, a process known as reapportionment. States then undertake the drawing of new geographic boundaries for political districts, a process known as redistricting. Districts must be geographically contiguous and have approximately the same populations. State governments determine the process for drawing district boundaries. In most states, the state legislature and governor decide on district boundaries, but in others the process is handled by a bipartisan commission.
When politicians are charged with drawing district boundaries, the party in the majority is likely to use this power to its advantage by drawing district lines that give it the chance to dominate the congressional delegation. The process of redistricting that creates geographic boundaries designed to give an electoral advantage to one political party is known as gerrymandering. (In the early 19th century, Governor Elbridge Gerry drew a congressional district to favor his party that looked like a salamander; "Gerry's salamander" gave rise to the term gerrymander.) Gerrymandering can be accomplished by following strategies called packing and cracking. Packing is the practice of drawing the lines of congressional districts to concentrate members of a particular group in one district to allow supporters of the other party to win neighboring districts. Another tactic is cracking, the practice of drawing the lines of congressional districts to split members of a particular group so they will not constitute a majority in any district. This manipulation is possible because Democrats are more likely to live in certain areas and Republicans in other areas, making it possible to divide one or the other party's voters.District lines have sometimes been challenged in courts. The U.S. Supreme Court has ruled against district lines drawn to minimize the electoral power of minority groups—a practice called racial gerrymandering. It has not moved against districts drawn on a partisan basis. Following the 2010 census, Pennsylvania Republicans drew district lines that favored their party and won victories in 12 of 18 House districts in the 2016 election despite the fact that Democratic candidates won just over 50 percent of the votes cast for congressional candidates in that election. When the Pennsylvania League of Women voters sued, charging that the district lines were the result of an unfair partisan gerrymander, the state supreme court agreed and drew new district lines. Those lines were expected to produce a more balanced congressional delegation—albeit one that still favored Republicans slightly.
Pennsylvania Redistricting Maps
Incumbents have a strong advantage in congressional elections. Midterm elections, however, can be used to gauge whether the current president has strong public approval or strong public disapproval. Popular presidents can help members of their party win open seats in Congress and sometimes even challenge incumbent members of the opposing party who are up for reelection. In contrast, unpopular presidents can contribute to their party's loss of seats in Congress and sometimes make incumbents in their own party vulnerable to challenges from opposing party candidates. Historically, the party of the president loses seats in the House in midterm elections. The drop in seats can be large enough to cause a shift in the majority in the House or Senate, which has an impact on the second half of the president's term.