The decision of the special prosecutor leading the involuntary manslaughter case against Alec Baldwin to step aside is a constitutional win for the actor — and all those brought to the bar of justice in New Mexico. It suggests the state’s case against the “Saturday Night Live” star for what happened on the set of “Rust” could need more rehearsing — because of one of the most important slabs of American bedrock.

The actor’s lawyers moved to replace that prosecutor, Andrea Reeb, because she also serves as state representative, having been elected in January. They argued that she constituted a one-person breach of separated powers. The actor insisted (and he was right as rain) that a “sitting member of the Legislature may not” — and here he’s quoting the New Mexico Constitution — “‘exercise any powers properly belonging to either the executive or judicial branch.’”

Before Tuesday, Ms. Reeb had been resisting. “I will not,” she boasted, “allow questions about my serving as a legislator and prosecutor to cloud the real issue at hand.” She came, we’d like to think, to grasp that the real issue requires adherence to the Land of Enchantment’s constitution. Now, she allows that the “best way I can ensure justice is served in this case is to step down so that the prosecution can focus on the evidence and the facts.”  

Mr. Baldwin’s camp argued that “allowing a single person” — in this case, Ms. Reeb —  to “exercise both legislative and prosecutorial power could taint prosecutorial decision-making.” That’s because the prosecutor could be driven not by impartial evaluation of the evidence but by “legislative interests.” Mr. Baldwin has long been a visible champion for left-wing causes. Charging and campaigning could commingle.  

Advocates for the movie star pointed to Article III of New Mexico’s charter, which ordains that “no person or collection of persons charged with the exercise of powers properly belonging to one of these departments,” meaning the legislative and executive, “shall exercise any powers properly belonging to either of the others.” This picks up on a concept most famously articulated in Article XXX of the Massachusetts Constitution.

“In the government of this Commonwealth,” the parchment, written by John Adams, says, “the legislative department shall never exercise the executive and judicial powers, or either of them: The executive shall never exercise the legislative and judicial powers, or either of them: The judicial shall never exercise the legislative and executive powers, or either of them: to the end it may be a government of laws and not of men.”

This is also marked in 51 Federalist, from James Madison’s own personal hand. It warns “against a gradual concentration of the several powers in the same department.” He adds that the “provision for defense must in this, as in all other cases, be made commensurate to the danger of attack.” In The Great Scalia’s memorable phrase, sometimes the attack is one where the “wolf comes as a wolf.”  

The reason we dilate on all this is the way it shows how the majesty of our constitutional principles are relevant to even the most tawdry cases. The charges New Mexico has handed up against Mr. Baldwin and armorer Hannah Gutierrez-Reed  are over the death of cinematographer Halyna Hutchins. Mr. Baldwin and Ms. Gutierrez-Reed are both charged with involuntary manslaughter, and both have pleaded “not guilty.” 

Prosecutors have dropped a charging “enhancement” that would have added years onto a potential prison sentence. The relevant portion of the New Mexico criminal code finds an instance of manslaughter if a “person is engaging in a lawful act but unintentionally kills someone by being negligent or not exercising due care.” It carries a sentence of up to 18 months behind bars. 

Mr. Baldwin, who was holding the firearm on October 21, 2021, when it discharged and killed Hutchins and injured the director, Joel Souza, claims that he did not pull the trigger and that he did not know that there was live ammunition in the gun. An FBI forensics report, however, found that the gun could not have fired “without a pull of the trigger.” The FBI used a hammer on the same gun wielded by Mr. Baldwin to reach that conclusion.  

What strikes us about the case is the closeness of all these details —  and thus the importance of having a prosecutor who is clear-eyed and without any kind of interbranch bias. It is something to think that one of the most famous actors in the world will seek justice by clinging to words scratched out by the man who would become America’s first vice president and second president nearly 250 years ago.