The basic mechanism
The US president is not elected by direct popular vote. Instead, each state is allocated electoral votes equal to its total congressional representation (House members plus two Senators). California has 54 electoral votes; Wyoming has 3. There are 538 electoral votes in total; 270 are needed to win the presidency.
Winner-takes-all
In 48 of 50 states (plus the District of Columbia, which has 3 electoral votes), the candidate who wins the popular vote in that state receives all of its electoral votes. Maine and Nebraska allocate their electoral votes proportionally. This winner-takes-all dynamic means the margin of victory in a state is irrelevant — winning California by 5 million votes or 1 vote produces the same 54 electoral votes.
Why this produces anomalies
The practical consequence is that presidential campaigns focus intensely on a small number of "swing states" — states where neither party has a reliable majority — while candidates invest little in safe states they expect to win or lose comfortably. It also means a candidate can win the presidency while losing the national popular vote. This has happened five times: in 1824, 1876, 1888, 2000 and 2016.
The reform debate
Reformers advocate either abolishing the Electoral College (requiring a constitutional amendment, which would need ratification by three-quarters of states) or the National Popular Vote Interstate Compact, in which states pledge their electoral votes to the national popular vote winner once enough states have joined to reach 270 electoral votes. The compact has been adopted by states with 209 electoral votes as of 2024.