One important aspect of this development is its role in the attitude of the United States Supreme Court. It interprets the ‘cruel and unusual punishment’ clause in article 8 of the Bill of Rights with reference to ‘evolving standards of decency’. These are assessed in large measure with reference to the behaviour of the state legislatures.
For example, in 2005, when the Court declared executions of persons for juvenile offences to be unconstitutional, it relied on evidence of legislative repeal of such measures, even though a considerable number of states in the United States continued to retain such legislation (after all, if they did not, what would be the point of a challenge in the Court).
Change is underway in the United States, and the movement is all in one direction. Abolition of the death penalty is a matter of time. It will probably come sooner than most people expect.
Thanks to Alfred de Zayas.