The new budgetary imperative is set to keep US execution rates on their decade-long downward path, a trend precipitated by the advent of DNA evidence and a widening acceptance of the judicial system's fallibility.

Last year 46 people were executed in America, less than half the number in 1999, the peak in the three decades since the Supreme Court reinstated capital punishment.

Death sentences are also declining sharply: last year 114 people were sentenced to die, a third of the number that were being sentenced in the mid-'90s.

''The death penalty is putting millions [of dollars] into getting one execution per state per year, if that,'' says Richard Dieter, of the Death Penalty Information Centre. ''It's just totally symbolic, totally political and that's an expensive thing to keep operating just because it plays well in soundbites.''

Five people have been executed so far this year, including a Missouri man on Wednesday who raped and killed an 11-year-old girl in 1991.

But another six cases have so far been stayed, some because of challenges to lethal injection protocols, while a second Missouri man had his sentence commuted after supporters brought forward evidence of his possible innocence.

Politicians are citing moral objections and mounting popular opposition to the death penalty, but increasingly they are advocating a pragmatic response to yawning state deficits, too.

Studies underscore the huge cost of killing convicted criminals as appeals and legal challenges draw out the process, sending bills skyrocketing. And new anxieties about getting the verdict absolutely right are adding further to the expense by pushing the average time that condemned prisoners spend on death row to 13 years, compared with half that in the 1980s: so long, in fact, that since 1983 more than 400 death row inmates have died of natural causes while waiting to be executed.

A Maryland study suggested that a death sentence - with all the trials, appeals and incarceration that it entailed - cost the state $US3 million, and more if the prisoner failed to be executed, whereas a life sentence cost $US1 million. An Indiana study put the cost differential as high as 10 times, while an investigation by New Jersey, which abolished the death penalty in 2007, revealed the cumulative effect: it found that death sentences had cost taxpayers an extra $US253 million since 1983.

''That's not the end of the story,'' Dieter says, ''but it is, in a time of financial crises, a foot in the door in terms of raising this issue. If you say 'I have a proposal to save the state millions of dollars', then you're going to get a hearing, at the very least.''

With cost a new wedge for campaigners, some state attorneys appear to be demanding the death sentence in fewer trials. And cost was clearly a factor in New Mexico's decision two years ago to ditch the death penalty, similarly in Illinois, where state legislators recently voted it out for new cases. (The Governor of Illinois is mulling whether to sign the bill into law.)

A similar move in Connecticut in 2009 was vetoed, but the incoming Governor, Dan Malloy, a lawyer and former prosecutor, has said he will sign the bill. Maryland's recently re-elected Governor has proposed an end to capital punishment in his state, where just five people have been executed since 1976, the last in 2005.

Should all three states follow through, it would take to 18 those that have axed capital punishment. Of the 32 remaining states, 20 have executed a total of 10 people in the past two years, and several states have not had an execution in more than five years.

A recent decision by some foreign-owned drug companies to no longer make sodium thiopental available for executions has cast additional doubt over the system, although some states are considering previously endorsed - and locally made - alternatives.

Despite the obstacles, political and assorted other interests continue to coalesce around the death penalty, affording it a notoriety within the American psyche, while its imperfections guarantee that it continues to be mired in controversy.

Critics complain of its arbitrariness: about one in 110 homicides attract the death penalty. And myriad studies expose its inequity on race: killing a white person was three times more likely to elicit the death penalty than killing blacks or Latinos, a recent study found. At play are assorted tensions.

David Garland, a professor of law and sociology at New York University and an expert on the death penalty, says ''politicians give voters what they want by enacting capital punishment statutes even when they will never be enforced, [and] prosecutors use the threat of a death penalty as leverage to elicit plea bargains and co-operation''.

Increasingly at play, too, are a reluctance among victims' families to clamour for an eye-for-an-eye, in fear that long, drawn-out legal appeals will anchor their hurt for years to come. Prosecutors increasingly are urging life sentences ahead of the death chamber, as a result.

DNA, however, has been the real ''game changer'' that has shifted the balance of opinion against the death penalty, featuring in 17 of 90 death row exonerations since the early 1990s and fuelling concerns about innocence in the minds of the public.

Even in Texas, noted as the ''perennial leader in executions'', DNA evidence and other investigation errors are giving authorities pause to think. DNA tests have recently shown that a single strand of hair, which placed Claude Jones at the scene of a murder, did not belong to him. But Jones was executed in 2000.

Texas, responsible for nearly four out of every 10 executions since 1976, is now supporting a state-wide DNA case review through the not-for-profit Innocence Project of Texas. While not confined to death penalty cases, the review comes after more than 40 convictions for a variety of crimes in Texas were overturned by DNA evidence.

Should more and more states opt out of the death penalty, some opponents contemplate the Supreme Court ruling the provision a violation of the Eighth Amendment, which bans ''cruel and unusual punishment''.

That may be fantasy and many years down the road, but observers such as Dieter are adamant that the current trends are ''not a blip''. ''They're a constant. They're a change in the popular view of the death penalty. It's not opposition, but it is scepticism, a frustration with it, a disbelief that it can really be fixed.''