The most insidious explanation for Democrat losses is "the Southern Strategy", which is a severe insult against all Southerners as ignorant racists. It is one of the most hyped theories around, but it is not a given that it even exists. It traces to the musings of one Republican who was a racist, but doesn't trace to any actual proof that any actual strategy even exists in the first place. All evidence for it is circumstantial - that since the late 60s the Solid South stopped voting Solid Democrat and started voting Republican a lot more often.
The Southern Strategy is allegedly based on the Civil Rights Act of 1964 and the fact that it was signed by a Democrat president, even though it was put on his desk because Congressional Republicans pushed it through. Key votes are examined such as when Goldwater voted for every Civil Rights Act before the 1964 one, and key votes are ignored such as when Goldwater's opponents voted against every Civil Rights Act before the 1964 one.
The theory states that because a Democrat president signed the Civil Rights Act, the ignorant and racist South turned Republican. The facts do not support that assertion.
Democrat Jimmy Carter's won victories in every Southern state except for Virginia and Oklahoma in the 1976 Presidential election, years after the alleged emergence of the Southern Strategy.
Democrat Bill Clinton was able to win five southern states twice (Louisiana, Arkansas, Tennessee, Kentucky, West Virginia) and two states once (Georgia in 1992 and Florida in 1996). Virginia, Texas and North Carolina were won by the Republican candidates by significantly smaller margins than usual.
The first Southern state to give the GOP control of both its governorship and its legislature was Florida. It did not do this until 1998. Florida has an atypical population for a Southern state, with a large retiree population from northern states and also a large Cuban population that leans Republican due to a shared opposition to Fidel Castro.
Georgia did not elect its first post-Reconstruction GOP governor until 2002. Until 2005, Louisiana had been represented since Reconstruction only by Democratic Senators. Arkansas has two Democratic Senators, a Democratic governor, three out of four of their U.S. representatives are Democrats, every statewide office is held by a Democrat, and their state legislature is Democratic. Tennessee and North Carolina have a majority Democratic delegation in the U.S. House of representatives. Mississippi has a house delegation that is evenly split between Democrats and Republicans.
It seems that the "Southern Strategy" which is quoted so often is more the musings of one particular racist Republican and not a Republican Party policy. Nixon is supposed to be the epitome of the evil American politician, but he is still just one person.