The US state of Texas is preparing to execute its 500th convict since the death penalty was restored in 1976, a record in a country where capital punishment is in decline elsewhere.

Iin the absence of a last minute pardon, 52-year-old Kimberly McCarthy will receive a lethal injection in Huntsville Penitentiary for the 1997 murder of 71-year-old retired college professor Dorothy Booth.

"What we do is we carry out court orders," said Jason Clark, spokesman for the Texas Department of Criminal Justice. "It's our obligation to carry this execution out."

Activists opposed to the death penalty are due to gather at the red brick state prison, known as the "Walls Unit," to mark the milestone with a protest against a punishment they regard as a holdover from another age.

In 1976, the US Supreme Court lifted a moratorium on the use of the death penalty and since that date 1,336 have been executed across the country, more than a third of them in Texas alone.

"It is obviously still the leader of executions in the nation, but it is limited to a handful of counties," said Steve Hall of the StandDown Texas Project, which has campaigns for a new moratorium.

"Texas leads with the number of executions and death sentences but there's no doubt you're seeing the same trend to decrease that you see nationally."

Richard Dieter of the Death Penalty Information Center, an academic watchdog, agreed.

"Despite this major milestone, we expect the total number of executions to be less than last year and a new drop in death sentences," he said.

According to DPIC's figures, there are 3,125 convicts on death row in the United States and, if Wednesday's execution goes ahead, McCarthy will be the 17th prisoner put to death in the first six months of 2013.

But numbers are dropping: 43 people were executed in 2012 down from a peak, in 2002, of 71.

American juries are also imposing capital punishment in fewer cases, with only 78 death sentences last year, down by around three-quarters since the 1990s -- although violent crime is also down.

And, while 32 of the 50 US states still have the death penalty on the books, many have imposed a de facto moratorium, with few or none of the executions carried out and convicts languishing on death row.

Activists like Dieter say this shows that these states will eventually formally abolish the death penalty, but supporters of the ultimate penalty note that it retains the support of American voters.

"By measurements like the number of executions, death sentences and states, the death penalty is in decline," admitted Robert Blecker, a professor at New York Law School.

"But, in terms of the popular support, that is fairly constant. It is not in decline," he said, noting that the proportion of voters backing execution always increases in the wake of "egregious" crimes.

Opinion polls consistently show that between 60 and 65 percent of Americans back the death penalty, indicating that support goes beyond the roughly 50-50 left-right divide in US electoral politics.

High-profile recent crimes like the gun massacres in a Connecticut school and a Colorado cinema, and the bomb attack on the Boston marathon, will only increase public support for capital punishment, Blecker added.

Nevertheless, abolitionists like Gloria Rubac of the Texas Death Penalty Abolition Movement see what she calls "light at the end of the tunnel."

She told AFP the 142 suspects who were condemned to death but then cleared on appeal have become "ambassadors for change" campaigning against death sentences, which are now largely concentrated in six southern states.

For some, there is also an economic argument. Contrary to popular perception, it could cost more to execute someone than to hold them for life, once the extra legal, security and logistics costs are added.

The Death Penalty Information Center estimates that each death penalty case in Texas costs taxpayers between two and three million dollars, or around two to three times more than imprisoning someone for 40 years.