Can a president be forced to testify?
While the Supreme Court has never definitively ruled on the subject, the answer appears to be yes.
The question was tested during the Watergate scandal in 1974, when justices held unanimously that a president could be compelled to comply with a subpoena for tapes and documents. After the ruling, President Richard Nixon turned the materials over to prosecutors and then resigned.
Twenty-three years later, in allowing Paula Jones’ sexual harassment suit to go forward against President Bill Clinton, Justice John Paul Stevens wrote: “We have made clear that in a criminal case the powerful interest in the ‘fair administration of criminal justice’ requires that the evidence be given under appropriate circumstances lest the ‘very integrity of the judicial system’ be eroded.”
In the same case, Stevens also said that presidents have given testimony and produced documents often enough that “such interactions … can scarcely be thought a novelty.”