So protecting our borders and following the Constitution is ruining America? I guess we've read different history books.
Perhaps we have. Have you read the ones where, in nearly every generation, immigration has been seen as a threat to the stability of the nation? In our history, reaching back even before the founding of the Republic, there have always been alarmists who claim that "real Americans don't let non-Americans become Americans."
Back in the 1740s, for example, Ben Franklin warned us all about the influx of Germans who were coming into the nation, claiming they would ruin it all for good, upstanding, God-fearing Englishmen.
In the 1850s, Pat Buchanan's forebears, Irish fleeing the potato famine, were the new menace. Then it was the Polish, the Italians, Russian Jews, Eastern Europeans, etc.
In each instance the "menace" was soon assimilated, usually by the second generation, and then a new menace arose.
That's why I say Buchanan is "anti-American" because he like you completely ignores or denies the fact that steady immigration has always been the American way, that despite repeated warnings over the last three hundred years of the downfall of society as the result of each new "invasion" nothing of the sort ever happened, and that the more insular societies like Japan or China are the ones that tend to stagnate.
So yes, Buchanan is "anti-American" by the REAL definition of "American." You are not permitted to adjust reality to suit your prejudices.