For most of it's history, Whites were the dominant majority of the population in the United States. They are still the majority, but the country is changing and there is no doubt that by the middle of this century that will no longer be the case.
Currently Whites compose about 62% of the population -- 26.3 points less than in 1940. Meanwhile, Blacks, Hispanics, and Asians all compromise a growing percentage of the population (with Hispanics being the fastest growing portion of the population). There are already seven states where whites no longer are a majority of the population, and several more are soon to follow.
This demographic change has the Republican Party worried. And they should be worried, since the modern GOP bases much of its policies on maintaining White privilege and opposing immigration. If they continue those bigoted policies, which it seems likely they will do for the foreseeable future, they are destined to a less relevant status with each passing election.
Instead of changing their policies, they have decided they can stay relevant by building a wall and cutting off (non-white) immigration. It is a policy destined to fail. They don't realize that the growth in non-white population percentage is mainly due to children being born in the United States, rather than immigration. Closing our borders will not stop the changing demographic (and probably won't even slow it down).
The charts below show the percentages of the White and Non-white populations in each state.