Whatever the GOP is doing is working. They have bigger majorities than anytime since 1928
Dont forget the "GOP" isnt just President. It is also the local recorder of deeds, the city council, the state legislator. By that measure the GOP is the majority party. The Dems are in a similar position to the GOP with Ford in 1975. They have the WH but nothing else.
They've won the popular vote once in the last six elections. It's pretty clear that they start in a hole in a nationwide high turnout election. Their success in other areas has come from low turnout midterms and gerrymandering. Of course, at the end of the day it doesn't make a difference, the people who hold the offices still hold them. But it's difficult for the GOP to claim a greater mandate than the Democrats until they can win a presidential election comfortably (which they haven't done since 1988).
Their losses in presidential elections explains their successes in midterm elections, which are simply biased against the party in the White House, not inherently pro-GOP.
Poll
