Nate Silver of FiveThirtyEight has calculated that for one candidate, winning isn’t enough.
In order for Joe Biden to have just a 46% chance of becoming President, he needs to win the popular vote by 2% to 3% (about 3 million votes).
A 3% to 4% lead in the national popular vote would give Biden only a 74% chance of becoming President.
The reason for this is that state “winner-take-all” laws award all of a state's electoral votes to the candidate receiving the most popular votes in each state.
If a candidate can eek out a small win in one or more closely divided states, it really doesn't matter what the rest of the country thinks under the current state-by-state winner-take-all method of awarding electoral votes.
- In 2016, Donald Trump became President even though Hillary Clinton won the national popular vote by 2,868,518 votes. Trump won the Presidency because he carried Michigan by 11,000 votes, Wisconsin by 23,000 votes, and Pennsylvania by 44,000 votes. Each of these 78,000 votes was 36 times more important than Clinton's nationwide lead of 2,868,518 votes.
- In 2000, George W. Bush became President even though Al Gore won the national popular vote by 537,179 votes. Bush won because he carried Florida by 537 votes. Each of these 537 votes was 1,000 times more important than Gore's nationwide lead of 537,179 votes.