U.S. Census Bureau

The US Presidential election takes place every 4 years and Congressional election happens every 2 years. National elections generally fall into two categories: elections where congressional seats are the highest offices decided and years where voters decide on the office of the President and congressional seats.To avoid confusion with presidential elections, the source refers to non-presidential year elections as “congressional elections" and the data is published at 4 years interval Reference link: https://www.census.gov/library/publications/2015/demo/p20-577.html