If the Democratic and Republican presidential nominees wanted to win a national mandate, it’s obvious that they would have to win a national popular vote plurality.
In every country, the head of government seeks a national victory. Prime Minister Boris Johnson, elected on perhaps the narrowest base of voters in any country, is trying for a bigger mandate and clear voter support of Brexit at this moment.
Even in totalitarian systems like Russia the leader wants to proclaim a national victory. Vladimir Putin obtained, he said, 77% of the national vote when he won his fourth term (second consecutive) in March 2018.
The nominees of the two major, national parties surely understand that a national mandate would give them much more political capital than is obtained by losing the national vote yet still claiming the presidency by dint of having won by a few thousand votes in, say, Wisconsin and Omaha, and squeaking to a total of 270 electors.
As players in a game, the two major nominees have an option to change the rules for winning. They can agree that they will instruct their electors to vote for the national vote winner. Then they have to demonstrate they are serious by campaigning everywhere, seeking every vote they can get regardless of geography.
In order to ensure that neither backtracks on the agreement, the two parties can have their electors swear a written and publicly videoed oath to vote for the national popular vote winner.