The "Great Replacement" theory involves much more than concerns over potentially skewed voting demographics, which could possibly favor Democrats. We are seeing several posters on here, who are trying to sanitize the theory, by only focusing on that one component of it.
It also involves the belief that black and Latino immigrants from Mexico, Central and South America, will be taking low-paying, manual labor jobs away from unskilled workers, who were born and raised in the United States. This theory is xenophobic in nature. It is a fear-mongering ploy, intended to stir anti-immigration sentiment among workers who comprise the lower end of the American socio-economic working class.