Ex parte Harding was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner who was being held in federal custody. The case arose when a prisoner, William Harding, was arrested by federal authorities in the District of Columbia and held in federal custody. Harding then filed a petition for a writ of habeas corpus in the Supreme Court of the District of Columbia, seeking to be released from federal custody. The Supreme Court of the District of Columbia granted the writ, and the federal government appealed the decision to the Supreme Court of the United States. The Supreme Court of the United States held that the Supreme Court of the District of Columbia did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the power to issue writs of habeas corpus was a power reserved to the federal government, and that the state court did not have the authority to interfere with the federal government's power to detain prisoners. The Court also noted that the writ of habeas corpus was a fundamental right that could not be taken away by the state court. In conclusion, the Supreme Court of the United States held that the Supreme Court of the District of Columbia did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the power to issue writs of habeas corpus was a power reserved to the federal government, and that the state court did not have the authority to interfere with the federal government's power to detain prisoners.

In Ex Parte Harding, the Supreme Court was asked to decide whether a writ of habeas corpus should be issued for an individual who had been convicted in a state court. The majority opinion held that the writ should not be issued because it would interfere with the jurisdiction of the state court. Justice Field dissented from this decision and argued that federal courts have authority to issue such writs when necessary to protect individuals' constitutional rights. He further argued that if there is any doubt as to whether or not those rights were violated, then it is up to federal courts—not state courts—to make such determinations. In his view, allowing states exclusive control over these matters could lead to injustices and deprive citizens of their right under Article IV Section 2 of the Constitution which guarantees them due process protections against arbitrary action by government officials.