Paul Starr contends that in the 2018 Congressional elections:

“The Democrats who flipped seats did so mostly in suburban districts where they attracted votes from independents and Republican moderates in what was an exceptionally strong year for Democrats. Many of the successful candidates were recruited to run precisely because they would appeal to moderates.”

The House races are the closest available proxy for a national presidential campaign. If the nominees had to win the national vote, they would ally with their party’s House candidates in every district, open get-out-the-vote offices with their House candidates, appear with them, and advertise with them.

Just as the Democrats won the House majority mostly because they attracted moderates, the presidential nominee also would have to attract moderates.

Requiring a national popular vote to elect the president would produce more moderate nominees, and the winner would be more moderate, than the current system which hinges the entire outcome on a handful of states not reflective of the demographics of the country as a whole.