Control of the House and Senate

The House is the simpler chamber: 435 districts, every seat contested every two years, and 218 seats for a majority. Districts are drawn by states after each census and, in many states, redrawn between censuses through litigation or legislative action - which means the map a market is pricing may not be the map that existed when the last election was held.

The Senate works on a different clock. Senators serve six years and the seats are divided into three classes, one of which is contested in each even year. Which states appear on the ballot in a given cycle is therefore fixed far in advance, and it matters enormously: a cycle where one party is defending seats in hostile states is structurally hard for that party regardless of the national mood.

Majority also means something different in each chamber. The House majority controls the floor almost completely. The Senate majority controls the calendar and committees, but most legislation still needs sixty votes to reach a final vote, so a bare Senate majority delivers confirmations and budget measures rather than a legislative programme.

The most market-relevant power is fiscal. Appropriations originate in the House, and a chamber controlled by the party opposing the president is where shutdown and debt-limit standoffs come from. Whether divided government exists at all is decided by these races.

The Senate's distinct power is advice and consent: cabinet officials, agency heads, judges and central bank governors all require confirmation, and the majority controls whether nominations get a hearing at all. Control of that chamber is effectively control of the personnel of the executive branch.

Both chambers control investigations and subpoenas, which is a slower channel but a real one - hearings move policy expectations and occasionally single companies. And control determines what never happens: a bill the majority does not want simply does not reach the floor.

Federal elections fall on the Tuesday after the first Monday in November of even years. Primaries run from March through September depending on the state, and the primary calendar is where candidate quality is determined - a factor that has decided more Senate races in recent cycles than the national environment did.

Between elections the arithmetic moves. Members resign, die, or take executive appointments; special elections fill the seats months later, and in the interim a majority can shrink to a single vote. In the Senate, most states allow the governor to appoint an interim replacement, so a vacancy can change the balance overnight without any election at all.

Redistricting is the House-specific variable. Maps are redrawn after each census and then contested in court for years afterwards, with rulings that move several seats at a time. A map struck down in spring can change the expected seat count for that November.