Presidential elections in the United States are decided by electors allocated among the states rather than by a single national tally. The allocation formula is simple, and its consequences follow directly from that simplicity.
The formula mirrors Congress
A state receives electors equal to its number of House members plus its two senators, so representation combines a population-based element with an equal-state element.
Because every state has two senators regardless of size, smaller states carry slightly more electors per resident than larger ones.
The District of Columbia receives electors through a constitutional amendment, capped at the number held by the least populous state.
The compromise addressed several disputes at once
The framers considered election by Congress, which raised concerns about executive independence, and direct popular election, which faced objections about information and coordination across a large country.
The electoral mechanism also resolved a dispute between states over how population would count for representation, a question tied directly to slavery at the time.
The result was a compromise assembled late in the convention rather than a design derived from a single theory of representation.
Winner-take-all is a state choice
The Constitution leaves the manner of appointing electors to state legislatures, and nearly all states have chosen to award their entire slate to the statewide plurality winner.
Two states allocate by congressional district with two electors going to the statewide winner, which shows the alternative is available.
Winner-take-all is not constitutionally required, and states have changed methods historically. It persists because it maximizes a state's leverage as a bloc.
Campaign attention follows the math
Under winner-take-all, an additional vote in a state already decided changes nothing, while a vote in a closely divided state can move the entire slate.
Campaigns concentrate spending and visits in a small number of competitive states as a direct consequence, and those states shift over decades as populations change.
This also explains why national popular vote margins and electoral outcomes can diverge, since votes beyond the threshold in a state are electorally inert.
Electors are people with a procedure
Slates of electors are nominated by state parties, and the winning slate meets in the state capital after the election to cast formal votes.
Those votes are transmitted to Congress, which counts them in a joint session governed by statutory procedure setting deadlines for state certification.
Most states bind electors to the popular result by law, and federal courts have upheld the authority of states to enforce such requirements.
If no candidate reaches a majority of electors, the House chooses the president with each state delegation casting a single vote. That contingency procedure has been used only twice in the country's history.