NPV--the National Popular Vote Initiative: Proposing Direct Election of the President Through an Interstate Compact [Updated October 29, 2019] [open pdf - 585KB]
From the Document: "The NPV [National Popular Vote] Initiative proposes an interstate compact, an agreement among the states that would effectively achieve direct popular election of the President and Vice President without a constitutional amendment. Each state that joins the NPV agrees to appoint electors pledged to the candidates who won the nationwide popular vote. Election authorities in the member states would count and certify the vote, which would be aggregated and certified as 'the nationwide popular vote.' Member state legislatures would then appoint the slate of electors pledged to the nationwide popular vote winner. They would do this regardless of who won the popular vote in their state. The compact would come into effect only if its success were assured--that is, only after states controlling a majority of electoral votes (270 or more) had joined the compact. States could withdraw from the compact, but if they did so within six months of a presidential election, the withdrawal would not take effect until after that election."
CRS Insight, IF11191
Congressional Research Service: https://crsreports.congress.gov/