In the case of Teofilo Medina, Jr. v. California in 1991, the U.S Supreme Court ruled that a defendant's competency to stand trial can be determined by state courts without violating due process rights under the Fourteenth Amendment. The court upheld California's standard for determining mental competence as constitutional and rejected Medina’s argument that it violated his right to fair trial because it did not require proof of his incompetence beyond a reasonable doubt. Instead, California law required only a preponderance of evidence showing he understood proceedings against him and could assist counsel in his defense - which was deemed sufficient by the court.

In the case of Teofilo Medina, Jr. v. California, Justice Blackmun dissented from the majority opinion that upheld California's standard for determining a defendant's competency to stand trial. He argued that this standard was constitutionally inadequate because it failed to consider whether a defendant has sufficient ability to consult with his lawyer and participate in his own defense - an essential aspect of ensuring fair trials under the Sixth Amendment right to counsel and due process clause of Fourteenth Amendment. Furthermore, he criticized the court for not establishing clear guidelines on what constitutes "rational understanding" or how states should evaluate it, leaving too much room for interpretation which could lead to inconsistent application across different jurisdictions.