Davidson v. Connelly was a United States Supreme Court case that addressed the issue of whether a state could constitutionally require a person to take an oath of allegiance to the United States before being allowed to practice law. The case was brought by William Davidson, a lawyer who had been denied admission to the bar of the Supreme Court of California because he refused to take the oath. The Supreme Court held that the California law requiring the oath was unconstitutional. The Court reasoned that the oath was a form of religious test, which was prohibited by the First Amendment. The Court also held that the oath was an unreasonable restriction on the right to practice law, since it was not necessary to ensure that lawyers were qualified to practice law. The Court's decision in Davidson v. Connelly was an important victory for the principle of religious freedom. It established that the government could not require individuals to take an oath of allegiance to the United States in order to practice a profession. This decision has been cited in numerous subsequent cases involving the First Amendment.

Justice Field delivered the dissenting opinion in Davidson v. Connelly, arguing that Congress had no authority to pass a law which would interfere with the rights of citizens under state laws. He argued that while Congress could regulate interstate commerce and other matters related to it, they did not have the power to dictate how states should treat their own citizens or what punishments they should impose for crimes committed within their borders. Furthermore, he argued that if such a law were allowed to stand then it would be an encroachment on state sovereignty and undermine its ability to govern itself as provided by the Constitution. In conclusion, Justice Field believed that allowing this federal statute was unconstitutional because it violated both due process and states' rights guaranteed by the Tenth Amendment of the United States Constitution.