The root of MAGA is the same as the root of “America First” (the early 20th century version): supposed humiliation/cuckholding/replacement of the white man.
For the first America First, it was rooted in the lingering animosities of the Civil War by “Southern Bourbons”. For MAGA (which is also a stolen slogan, btw), it was in the demographic decline of white births in the 1970s to below population replacement rates.
This is what happens when you have a two-party system and both parties are right wing.