The history of the United States Congress refers to the chronological record of the United States Congress including legislative sessions.
The Constitution defines the Senate as having two senators for each state in the Union. The size of the House of Representatives is based on the number of states and their populations. The numerical size of the House is set by law, not by the Constitution. The House grew in size as states were admitted throughout the 19th century, and as the nation grew in population. Since the Constitution allows for one representative for as few as 30,000 citizens, Congress passed new, higher limits for the House, which grew in size until a law passed in 1911, based on the National Census of 1910, established the present upper limit of 435 members of the House. Since the House's size was fixed but the population kept growing, instead of a congressperson representing only 30,000 citizens (as the Constitution had previously established), a congressperson represents 600,000 and more persons.
In different periods of American history, the role of Congress shifted along with changing relations with the other branches of government, and was sometimes marked by intense partisanship and other times by cooperation across the aisle. Its relations with the other branches of government have changed over time. Generally Congress was more powerful in the 19th century than in the 20th century, when the presidency (particularly during wartime) became a more dominant branch.
One analyst examining Congressional history suggested there were four main eras, with considerable overlap, and these included the formative era (1780s–1820s), the partisan era (1830s–20th century), the committee era (1910s–1960s), and the contemporary era (1970s–today).