The US Senate has 100 members — two from each of the 50 states, regardless of population. Wyoming (population 580,000) has the same Senate representation as California (population 39 million). This equal-state representation was the "Great Compromise" of the 1787 Constitutional Convention, giving small states parity with large states. The practical effect is that Republican-leaning rural states are structurally overrepresented in the Senate relative to their population share, while Democratic-leaning urban states are underrepresented. Senators serve 6-year terms and can run for re-election without limit.

Senators are elected statewide by all voters in their state — making Senate elections more expensive and harder to gerrymander than House elections (you cannot gerrymander a whole state). Senate races are among the most expensive elections in the country: competitive races regularly hit $50-100M+ in total spending by campaigns and outside groups. The statewide electorate is more diverse than any single congressional district, which means Senate candidates must build broader coalitions than House members representing specific geographic slices.

Senate seats are divided into three "classes" that were established at the founding to stagger elections and prevent the entire Senate from turning over at once. Approximately one-third of seats face election every two years, coinciding with every House election and every presidential election alternating. This staggering means that even a massive wave election can only affect the seats in the class up that year.

The 2026 Class II map is structurally favorable to Democrats in terms of seats at risk. Republicans defend 22 Class II seats; Democrats defend only 13. However, raw numbers don't tell the whole story: many of those 22 Republican seats are in deep-red states (Texas, Tennessee, Idaho, Indiana, etc.) that Democrats cannot realistically contest. The competitive pool shrinks to 5-7 seats on each side.

The Senate majority threshold varies depending on what the majority is trying to do. For most legislation, the filibuster requires 60 votes to cut off debate (invoke cloture) before a final vote can be held. This means the minority party (currently Democrats with 47 seats) can block most legislation by simply not providing the 13 votes needed to reach 60. The result: most major legislation requires bipartisan support, or it cannot pass.

Exceptions to the 60-vote requirement include: budget reconciliation (51 votes, used for tax and mandatory spending changes), presidential nominations (51 votes since the "nuclear option" was triggered in 2013 for executive/judicial and 2017 for Supreme Court), and procedural votes like the organizing resolution that determines committee ratios and chair assignments. The 51-vote threshold for nominations explains why the Republican Senate majority has moved quickly on Trump nominees despite Democratic opposition. The reconciliation exception explains why the "One Big Beautiful Bill" is moving through the Senate despite unified Democratic opposition — it requires only 51 Republican votes, which Republicans have with 53 seats, and can afford to lose 2.