As the chart below shows, voter turnout can increase drastically if a race is closely contested and people know their vote is likely to matter. A close Senate race in Texas and a close gubernatorial race in Georgia drove turnout up 14 and 21 points, respectively, above the average:
So if the presidential candidates competed to win the national popular vote, then every voter in every state would know their vote mattered. Turnout on average would go up in the 40 states currently ignored by the two parties’ candidates. An increase of 14 to 21 points would translate to at least 20 million more votes.
The two parties would have to reshape their policies, reconsider their coalitions, and perhaps change their nominating rules in order to capture a winning share of the huge influx of participation.