Probably about 15% ~ no more than that ~
12 percent of Americans are foreign born.
23 percent of Americans have at least one foreign born parent.
Something around 50 percent of the remainder are third generation americans, having at least one foreign born grandparent.
12+ 23 + 50 = 85 percent or so. :)
I’m curious why you believe this to be so low? You’d be stripping citizenship from Americans with only 3 grandparents or 7 great grandparents born in America.
Is this really the hill you want to die on?