Newsom has always been a vocal opponent of the death penalty, stating the practice discriminates against people of color, mentally ill people and poor people, and that it has put to death those wrongfully convicted of crimes.
In a statement announcing the executive order, due Wednesday, Newsom called the death penalty "premeditated and discriminatory".
"Our death penalty system has been - by any measure - a failure". "The intentional killing of another person is wrong".
Critics of the moratorium say Newsom's executive order ignores the will of the people since California voters in 2016 rejected a state ballot initiative to abolish the death penalty. Newsom said he anxious that the executions of more than 20 inmates who have exhausted their appeals would be resumed. Newsom is expected to say on Wednesday that he believes capital punishment to be costly and burdensome, and unevenly meted out to minorities and offenders with disabilities, the administration source said.
While campaigning for a measure to repeal the death penalty in 2016, he told The Modesto Bee editorial board he would "be accountable to the will of the voters", if he were elected governor. They also said five death-row inmates were eventually exonerated since 1973.
The moratorium will be in place for the duration of Newsom's time in office, the governor's office said.
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While the governor's move is certain to be challenged in court, aides cite his power to grant reprieves written into the state Constitution. At least 18 of the 25 people who were executed past year had significant evidence of mental illness, intellectually damaging brain injuries or chronic childhood trauma or abuse, according to a Death Penalty Information Center report released in December.
Newsom, whose state government is clashing with Trump over environmental regulations, Trump's promised border wall and plans for a bullet train, added he was still committed to working with the Trump White House on disaster preparedness.
He may have put a nail in that coffin with his decision to sign an executive order on Wednesday that would put a halt to the death penalty while he's governor.
"It's not supposed to be a weapon for blocking the enforcement of the law that the people have passed just because the governor disagrees with it", Scheidegger said.
A quarter of all those on death row in the United States are in California, according to the governor's office. Voters that year also favored fast-tracking the appeals process, speeding up convicted killers' time on death row, through approving Proposition 66.
Age and natural causes seem to have been a more effective death penalty in California: 79 death row habitants have died of natural causes since 1978, another 26 killed themselves.