How the National Popular Vote works:
The current winner-take-all method of awarding electoral votes is not in the U.S. Constitution.
The Constitution says: “Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors.”
The Constitution does not encourage, discourage, require, or prohibit the use of any particular way of awarding a state's electoral votes. The Constitution gives the states exclusive control over the choice of method of awarding their electoral votes, as illustrated by the fact that Maine awards 2 of its 4 electoral votes by district.
The National Popular Vote law will take effect when enacted by states with a majority of the electoral votes (270 of 538). Then, the presidential candidate winning the most popular votes in all 50 states and DC will get all the electoral votes from all of the enacting states. That is, the candidate winning the most popular votes nationwide will be guaranteed enough electoral votes to become President.
Under the National Popular Vote law, no voter will have their vote cancelled out because their choice differed from majority sentiment. Instead, every voter’s vote will be added directly into the national count for the candidate of their choice. This will ensure that every voter, in every state, will be politically relevant in every presidential election—regardless of where they live.