GENERATIONS OF Americans have understood that children born in the United States are entitled to U.S. citizenship, regardless of the nationality of their parents. When Congress revisits immigration reform this spring, however, legislation to repeal this historic rule is expected to play a central role in the debate.

Many Americans are angry about illegal immigration and believe birthright citizenship encourages it. Unsurprisingly, then, the idea of eliminating automatic citizenship for the children of lawful and unlawful aliens has gained remarkable traction around the country.

A resolution moving through the Georgia Legislature urges Congress to take such action. A coalition of conservative activists has proposed a grand immigration compromise: amnesty for illegal immigrants with relatives here now, but no birthright citizenship in the future. Texas lawmakers are even weighing legislation that would attack birthright citizenship indirectly by denying state and local government services to so-called "anchor babies" — children born in the U.S. to illegal immigrants.

In recent years, this effort has been bolstered by court briefs and congressional testimony from legal scholars. Even Richard Posner, the distinguished federal appellate judge, wrote in a judicial opinion that Congress can, and should, repeal birthright citizenship.

The breadth of support is surprising because the proposed legislation is plainly unconstitutional. Birthright citizenship is a constitutional right, no less for the children of undocumented persons than for descendants of passengers of the Mayflower.

The first sentence of the 14th Amendment, ratified in 1868, puts it plainly: "All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States." The primary purpose of this provision was to reverse the U.S. Supreme Court's infamous Dred Scott decision, which denied citizenship to U.S.-born people of African descent. But the amendment was drafted broadly to guarantee citizenship to virtually everyone born in the United States.

California Rep. Dan Lungren (R-Gold River) and other proponents of ending birthright citizenship claim that aliens — lawful and unlawful — are not "subject to the jurisdiction" of the U.S. because they swear no allegiance to the United States. But neither the text nor the history of the 14th Amendment supports this conclusion.

When a person is "subject to the jurisdiction" of a court of law, that person is required to obey the orders of that court. The meaning of the phrase is simple: One is "subject to the jurisdiction" of another whenever one is obliged to obey the laws of another. The test is obedience, not allegiance.

The "jurisdiction" requirement excludes only those who are not required to obey U.S. law. This concept, like much of early U.S. law, derives from English common law. Under common law, foreign diplomats and enemy soldiers are not legally obliged to obey our law, and thus their offspring are not entitled to citizenship at birth. The 14th Amendment merely codified this common law doctrine.

Members of the 39th Congress debated the wisdom of guaranteeing birthright citizenship — but no one disputed the amendment's meaning. Opponents conceded — indeed, warned — that it would grant citizenship to the children of those who "owe [the U.S.] no allegiance." Amendment supporters agreed that only members of Indian tribes, ambassadors, foreign ministers and others not "subject to our laws" would fall outside the amendment's reach.

The U.S. Supreme Court long has taken the same view. In 1898, the court held in United States vs. Wong Kim Ark that the U.S.-born child of Chinese immigrants was constitutionally entitled to citizenship, noting that the "14th Amendment affirms the ancient and fundamental rule of citizenship by birth within the territory . . . including all children here born of resident aliens."

The court has reiterated this view in subsequent decisions. In Plyler vs. Doe (1982), the majority concluded, and the dissent agreed, that birthright citizenship under the 14th Amendment extends to anyone "who is subject to the laws of a state," including the U.S.-born children of illegal aliens. And in INS vs. Rios-Pineda (1985), a unanimous court agreed that a child born to an undocumented immigrant was in fact a citizen of the United States.

Although the Constitution seems clear, Democrats in Congress might nevertheless be persuaded to repeal birthright citizenship as a bipartisan compromise to secure passage of a comprehensive immigration reform bill — in the hope that the provision would simply be struck down in court. Perhaps that explains why Senate Democrats quizzed Samuel A. Alito Jr. about the issue during his confirmation hearings. Stay tuned: Dred Scott II could be coming soon to a federal court near you.