IT WAS not up to President Obama to label the birther movement as racist in his extraordinary address on the subject last week, but plenty of commentary has done it for him. There can be no doubt that the lurid contempt shown to the president by antagonists who question his constitutional right to hold office is rooted in white-supremacist hysteria. The issue has never been the authenticity of documentation related to Barack Obama’s date and place of birth, which is why the production of birth certificates — first short, then long — has not stilled the controversy.

The issue has been his character as — well, as the issue of a Caucasian mother and an African father. An inch below the surface of this discussion is the perceived offense not just of blackness, but of miscegenation, that peculiarly demonic legacy of a slave system which took for granted the white owners’ sexual exploitation of slaves, while outlawing interracial sex. The biological fact of Obama’s existence, not the bureaucratic fact of government records, is what generates the lunatic rage.

Echoes abound in this affair of a very old story. First, an African-American is elected president, presumably opening a new era of racial equality. Then the racists push back with visceral denial that such a man is even eligible for the office. The pattern is well established. First, freed slaves are promised 40 acres and a mule, but then, forced into sharecropping, they are reshackled to white landowners by debt. First, the 13th Amendment to the Constitution abolishes involuntary servitude, but then Jim Crow laws reinstate it across the South. First, the civil rights movement trumpets the long-postponed end of black subjugation in America, but then a nationwide wave of draconian anti-drug laws sends people of color to prison with wild disproportion. The burden is always on blacks to prove what, in the case of whites, goes without saying. Prove innocence. Prove eligibility. Prove rights. Prove competence. Prove that proof is genuine.

But echoes in the birther contretemps go back even further than America’s sorry history. That word “birth’’ is the key, and what it opens is the door to the invention of racism in the first place. Whiteness as the positive norm against blackness as the negative norm repeats the bipolar social construction that occurred when the racial category of anti-Semitism replaced the religious category of anti-Judaism. Wrong birth replaced wrong belief. Modern racism, that is, began with fixations attached not to skin color or ethnicity, but faith.

In the beginning, Jews were rejected by Christians because they rejected Jesus. If they converted, underwent baptism, and accepted Jesus, they could become full members of the dominant society. But then, when Jewish conversions were systematically coerced by rulers and prelates, especially in 15th century Iberia, Christians found reason to doubt the sincerity of conversos. Anyone born a Jew was suspect, even after baptism. Birth, not belief, became the issue.

An odd foreshadowing of the Obama-birth affair occurred in 1546 when a converso priest was appointed to a high position at the cathedral in Toledo, Spain. The archbishop, suspecting the man’s Jewish origins, overruled the appointment on the grounds that he had “impure blood.’’ To head off any further such appointments, the archbishop issued the so-called Statute of Toledo, requiring “blood purity’’ — limpieza de sangre — of any candidate for office in the cathedral. The Inquisition extended such limpieza statutes to other institutions, with the result that persons of Jewish birth or even ancestry — whether baptized or not — were banned from universities, religious orders, guilds, and municipal offices.

The church itself eventually revoked blood purity regulations, but not before they had widely taken root in Europe. The institutionalization of this distinction by blood — by birth — assumes a biological divide of racial superiority and racial inferiority, and stands as a marker in human history. The construction of supremacy. Applied at first to Jews, the birth standard would soon enough (as Europe’s “age of exploration’’ accelerated) be applied to native peoples everywhere, beginning in Africa.

That this discussion occurs in the United States and is sufficient to prompt a response from an elected president is thus not nearly as astonishing as it seems, but it is far more shameful than any Republican has yet acknowledged.

James Carroll’s column appears regularly in the Globe.

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