All 435 seats in the House of Representatives are apportioned to the states based on the results of that decade's Census. Each congressional district is home to approximately 761,000 people—though that number can vary by tens of thousands of residents.

The 2020 Census apportioned 52 congressional seats to California's 39.5 million residents, while the six least-populous states—Alaska, Delaware, North Dakota, South Dakota, Vermont, and Wyoming—each received one at-large congressional seat. American Samoa, Guam, Northern Mariana Islands, Puerto Rico, the Virgin Islands, and Washington, D.C., each send one non-voting delegate to the House to represent their residents. These delegates have offices, speaking privileges, and committee assignments, but may not vote on bills on the floor.

Many congressional districts around the country are subject to intense gerrymandering—district boundaries that are drawn to include or exclude certain voters to achieve a desired outcome for one party. Gerrymandering has directly influenced control of the House of Representatives on numerous occasions, most recently in 2024.

Rural districts heavily favor Republican candidates, while urban districts and southern majority-Black districts largely favor Democratic candidates. Increased political polarization and gerrymandering have led to fewer battleground districts today than at any point in modern American history. The most competitive House races are often in the suburbs—and dependent on turnout rather than voters changing their minds.

After the 2024 election, Republicans held a bare majority with 220 seats, while Democrats held 215 seats.

The Senate is comprised of 100 members. Each state receives two senators regardless of its population. One-third of the Senate is up for election every two years. Vacancies are filled by the respective state's governor, and special elections are held to fill the remainder of the term.

The authors of the Constitution intended for the Senate to provide less-populous states a fair shake against domination by populous states in the House of Representatives. In practice, though, the chamber's composition and rules—such as the filibuster—have provided small states an oversized (and arguably unfair) veto on the rest of the country.

Control of the Senate has swung back and forth between the two parties since the political realignment of the southern states in the 1970s and 1980s. The most recent political realignment in the early 2010s gave Republicans an advantage in the Senate as a function of the sway held by small, rural states in the chamber.

After the 2024 election, Republicans held 53 seats and the Democrats held 47 seats. Only Wisconsin and Pennsylvania elected a divided Senate delegation; the remaining 48 states either sent two Republicans or two Democrats to the chamber.

The following chart shows the results of congressional elections between 1856 and 2024, which corresponds to the existence of the two-party dynamic between Democrats and Republicans. Elections before 1856 were relatively messy, drawn out, and resulted in more factions represented than a chart like this can legibly display.