What has been the single most important variable in determining the outcome of the election for members of the House of Representatives (in the last 30 years)?

In the last 30 years, the single most important variable to determine the outcome of an election for members of the House of Representatives has been incumbency.

This term is one that can mean several closely related things including something obligatory imposed as an obligation or duty but also lying, leaning, or resting on something else. The term can also mean that someone is currently holding a specified office such as using the phrase ‘incumbent Mayor.’ Instead of directly by election members of the House of Representatives are chosen by electors from each state.