That underscored the struggles that death-penalty states are having with lethal injection, particularly after European manufacturers, in recent years, stopped providing the drugs for executions. Some American states are now contemplating a return to the electric chair or even firing squads, two methods with their own troubled histories.

Then last month, the Supreme Court again narrowed the scope of allowable executions, holding that a simple I.Q. score of 70 was too blunt a standard for determining whether inmates had the intellect to defend themselves and accept ultimate responsibility for their actions. Previously, the court had barred capital punishment for some rapists, for the insane and for juvenile offenders, marking a slow but steady retreat from the ultimate penalty.

In an ABC-Washington Post poll released earlier this month, 60 percent of respondents said they backed the death penalty, down from a high of 80 percent in 1994. But for the first time in that poll, Americans given a choice between the death penalty and life in prison for convicted murderers preferred life, by 52 percent to 42 percent.

The debate on both sides of the Atlantic has revolved around similar issues: possibly innocent defendants, unequal application of the death penalty “and the barbarity of some executions,” said Richard C. Dieter, executive director of the nonprofit Death Penalty Information Center.

Abolition came sooner in European countries, death-penalty experts say, partly because of revulsion over the wartime use of capital punishment, especially in Germany, and because of differences in justice and political systems.

In most European countries, judges and prosecutors are appointed rather than publicly elected — a crucial distinction because many people favor the death penalty as a form of personal justice. And under multiparty parliamentary systems, some scholars say, socially divisive issues can more easily be managed without the fear, prevalent in a two-party system, of either side’s being tarred as “soft on crime.”

“In most places in Europe, the death penalty is abolished by passing a law,” said Anthony J. McGann, a political science professor at the University of Strathclyde, in Scotland, who has analyzed abolition patterns. “Passing such a law in the United States would require the House and Senate to agree, and a filibuster-proof majority in the Senate,” which he called “almost unthinkable.”