What about after the 2020 election? The Republican Party is likely to gain a greater advantage in the Electoral College due to the 2020 census than it already has. As the map below shows, the Democrats are likely to lose net five or six electors from states where they typically carry the plurality. These are RI, NY (2), PA, MI, IL, MN (?), CA (?), offset by gains of one elector in OR and CO. That is a swing of 10 or 12 electors in favor of the Republicans. Some think the Republican Party after what we presume is Donald Trump's last election (who knows for sure?) will then want to support the national popular vote. Those with this point of view have to explain why this party's professionals, donors, and future candidates will want to relinquish the electoral advantage.
It can be argued of course that the plurality will shift toward the Democrats in FL, TX, and AZ, but that sort of speculation assumes a blue wave washes over the national political landscape. It might be true of course. That is what Stan Greenberg thinks. But why would that cause professional Republicans to support the national vote as the way to choose the president? The answer might be that the party survives the possible blue wave by compromising on various divisive issues; by changing its positions to reflect the changing preferences and demographics of the country. On the other hand, if the parties remain approximately in the same position on the issues after 2020, then the Republicans will continue to believe that with their current views, so vividly presented by the president, they will probably carry FL, TX, and AZ, and that the electoral gains of these states will accrue to the advantage of their party.