The Democratic candidates have increasingly staked out opposition to the death penalty after decades of party stalwarts supporting it to appear tough on crime. Former Vice President Joseph R. Biden Jr. reversed course when he unveiled a criminal justice platform on Tuesday, joining nearly all of his opponents. He noted that 160 people sentenced to death since 1973 in the United States were later exonerated, a statistic that he repeated Thursday on Twitter, adding, “Because we can’t ensure that we get these cases right every time, we must eliminate the death penalty.”

Mr. Biden was the lead sponsor of the bill that became the Violent Crime and Law Enforcement Act of 1994. Some of the inmates now scheduled to be executed are eligible for the federal death penalty under that law.

In response to the Justice Department announcement, Representative Ayanna S. Pressley, Democrat of Massachusetts, introduced a bill that would abolish the federal death penalty.

The Supreme Court reinstated the death penalty in 1976, four years after effectively striking it down. Many states quickly adopted laws restoring capital punishment, though the federal government did not do so until 1988 for a few offenses and 1994 for many more.

Like the president, Mr. Barr has long supported capital punishment, including during his first stint as attorney general during the George Bush administration. “We need a death penalty to deter and punish the most heinous federal crimes such as terrorist killings,” Mr. Barr wrote in an Op-Ed in The New York Times in 1991 when he was acting attorney general. “That penalty would send a message to drug dealers and gangs.”

But public attitudes toward the death penalty have changed in the ensuing decades. Support for it went from nearly 80 percent in 1996 to a two-decade low three years ago, when just under half of Americans polled backed it for people convicted of murder, according to the Pew Research Center. Public backing of capital punishment ticked back up to 54 percent last year, the center found.

Capital punishment fell out of favor as researchers questioned whether it deterred people from committing heinous crimes and as more defense lawyers proved that their clients had been wrongfully convicted. Fewer than two dozen executions have occurred annually in the United States in recent years, down from a high of 98 in 1999, according to the Death Penalty Information Center.