In the United States v. Valenzuela-Bernal, 1981, the Supreme Court ruled that a defendant's Sixth Amendment right to compulsory process for obtaining witnesses in his favor was not violated when potential witnesses were deported before trial. The case involved two Mexican nationals who had been charged with transporting illegal aliens into the US and their claim that they were denied due process because some of those transported had been deported before they could be questioned as potential defense witnesses. The court held that simply showing that the government made it impossible for a defendant to call a witness is insufficient to establish a violation of the Compulsory Process Clause; rather, defendants must show both cause (that government conduct interfered) and prejudice (that there was reasonable likelihood these would have testified on behalf). In this case, since no plausible testimony from deportees was identified by defendants which could have affected outcome of trial or assisted in their defense strategy, no constitutional rights were violated.

In the dissenting opinion for United States v. Valenzuela-Bernal, Justice Blackmun argued that the majority's decision was too restrictive in its interpretation of a defendant's right to compulsory process under the Sixth Amendment. He believed that this ruling would unfairly burden defendants by requiring them to prove not only that they were denied access to potential witnesses, but also what those witnesses would have testified about and how it could have affected their defense. This standard is almost impossible for most defendants to meet because they cannot know what a witness might say until after they've had an opportunity to interview them. Furthermore, he pointed out that this approach contradicts previous rulings where courts presumed prejudice when there was government interference with a defendant’s preparation of his case or if evidence favorable to an accused was suppressed by the prosecution.