Gallup poll. The increasing number of states abolishing the practice and holding moratoriums may be linked to the decline in support for state-sanctioned killing:
Sixty percent of Americans say they favor the death penalty for convicted murderers, the lowest level of support Gallup has measured since November 1972, when 57% were in favor. Death penalty support peaked at 80% in 1994, but it has gradually declined since then.
...The current era of lower support may be tied to death penalty moratoriums in several states beginning around 2000 after several death-row inmates were later proven innocent of the crimes of which they were convicted. More recently, since 2006, six states have repealed death penalty laws outright, including Maryland this year.
Politics is a major dividing line in Americans' death penalty views -- 81% of Republicans currently favor it, compared with 47% of Democrats. Independents' 60% support matches the national average.
...Currently, 18 states do not allow the death penalty, and six of those bans have occurred since 2006. Six others instituted bans during the mid-1950s through the early 1970s, when U.S. support for the death penalty was lowest historically.