In the case of Cuyler, Correctional Superintendent, et al. v. Sullivan in 1979, the U.S Supreme Court ruled that a defendant's Sixth Amendment right to counsel is violated if their attorney has a conflict of interest that adversely affects their performance, even if the attorney was retained by the defendant rather than appointed by court. The case involved an inmate named Sullivan who had been represented at his murder trial by two lawyers also representing co-defendants in related cases. He claimed this constituted ineffective assistance due to conflicting interests among clients and sought habeas corpus relief (a legal procedure for claiming unlawful imprisonment). The Court held that defendants must demonstrate actual conflict affected representation quality; mere possibility isn't sufficient grounds for overturning conviction on Sixth Amendment grounds.

In the dissenting opinion for Cuyler v. Sullivan, Justice William Rehnquist disagreed with the majority's decision to extend Sixth Amendment protections to state criminal defendants who have retained multiple lawyers from a single firm. He argued that there was no evidence of actual conflict in this case and thus, it should not be presumed based on mere possibility. Furthermore, he contended that extending such constitutional protection would unduly burden the legal system by encouraging unnecessary litigation over potential conflicts of interest where none may exist in reality. He also expressed concern about undermining public confidence in defense attorneys' professional integrity by presuming they cannot manage their own conflicts of interest effectively.