In Dobbert v. Florida, the U.S. Supreme Court ruled in favor of Florida, upholding a death sentence despite changes to the state's capital punishment laws during trial proceedings. The defendant, Ernest John Dobbert Jr., was convicted for murdering his two children and sentenced to death under new legislation that had been enacted after he committed his crimes but before his trial began. He appealed on grounds that applying this law retroactively violated the ex post facto clause of the Constitution which prohibits punishing actions more severely than they were at their time of commission. However, the court held 7-2 that there was no ex post facto violation because although procedural methods changed between crime and conviction (from jury recommendation to judge discretion), standards for imposing death penalty remained consistent - only applied for aggravated circumstances or particularly atrocious crimes - thus not increasing punishment severity retrospectively.

In the dissenting opinion for Dobbert v. Florida, Justice Marshall argued that the death penalty is always cruel and unusual punishment in violation of the Eighth Amendment. He contended that it was not appropriate to apply new standards retroactively, as this would violate ex post facto laws which prohibit punishing actions under laws enacted after those actions were committed. In his view, when a state changes its law to make more severe punishments possible than were available at the time of an offense's commission, applying such increased penalties violates constitutional principles against ex post facto legislation. Furthermore, he expressed concern about arbitrariness in sentencing decisions and emphasized that capital punishment does not serve any valid penological purpose like deterrence or retribution.