PONTIAC, Mich. — It was a gut judgment, one made quickly, the prosecutor Karen D. McDonald said of her decision to charge Ethan Crumbley’s parents with involuntary manslaughter in the shooting deaths of four students at Oxford High School in suburban Detroit.

“I just instinctually knew there was just absolutely no way I was not going to prosecute them when I heard all the evidence,” Ms. McDonald said in an interview.

Ms. McDonald pointed to what she said is the chilling lead-up to the Nov. 30 massacre: how James and Jennifer Crumbley bought their son a 9-millimeter handgun as an early Christmas present, then failed to tell school officials about it, even after they called the parents to a meeting because a teacher saw their son’s drawing of a gun, a victim and the plea, “The thoughts won’t stop. Please help me.”

Her decision came despite pushback from some members of her staff at the Oakland County Prosecutor’s Office, she said. They cited significant legal hurdles and suggested that she hold off, fearing a defeat in court.

But, Ms. McDonald said, she is not afraid to lose.

“It’s the right thing to do,” she said. “And if the jury decides that they aren’t criminally culpable, I can live with that.” The couple’s defense lawyers have said that a fuller picture, one that absolves the parents, will emerge.

Ms. McDonald said she did not set out to break new legal ground — charging parents of a minor in a mass shooting is highly unusual — but she now views the case as potentially part of something bigger, a new frontline effort to promote responsible gun ownership. She calls it “a brand-new way of approaching school shootings,” one that she hopes can help course correct the system’s past failures in deterring the killings.

The Oxford case has catapulted Ms. McDonald, 51, from an obscure prosecutor in her first year on the job to a national figure, one who appears frequently on news programs making her case for tougher gun control.

Her decision to prosecute the Crumbleys resonates with victims, including Michele Gay, who lost her youngest daughter, Josephine, in the mass shooting at Sandy Hook Elementary School in Newtown, Conn., that killed 26 children and adults.

“We do have to put it all out on the table every time, as painful as it is, and look at every possibility that would have mitigated this, stopped this,” said Ms. Gay, who founded the organization Safe and Sound Schools after her daughter’s death in 2012.

Ms. McDonald could well emerge as a political force in Michigan, according to Richard Czuba, a political pollster in Lansing, Mich.

“Certainly she’s a name,” said Mr. Czuba, who said her response to the shootings “strikes a chord in households with children.”

He envisions Ms. McDonald as a potential successor to Dana Nessel, the state attorney general, when Ms. Nessel’s second term ends at the end of 2026.

But that’s a lifetime or two in politics years, and first Ms. McDonald must prove her legal strategies can work, including her unusual decision to charge Ethan Crumbley with not only first-degree murder, but also terrorism causing death.

During the interview at the sprawling county government complex in Oakland County, Mich., the state’s second largest county, Ms. McDonald said she intends to be in court as part of the county’s prosecution team when the couple goes to trial. Both the Crumbleys, who each face up to 60 years in prison, and their son, who faces life in prison, have pleaded not guilty.

Earlier in the day, one in which a funeral was held for the youngest victim, 14-year-old Hana St. Juliana, Ms. McDonald toured the crime scene at Oxford High School.

The scene, Ms. McDonald said, revealed “calculated horrific behavior,” describing how one of the four victims was killed, execution style, in a bathroom. Seven other people were injured, including a teacher.

The case against the parents will depend on whether prosecutors can show that the parents were “aware not just generally that their son was troubled but aware that he would or could do this — shoot up that school,” said Eve Brensike Primus, a professor at the University of Michigan School of Law. “That requires a level of mental culpability that is more challenging for a prosecutor to prove.”

While parents have been charged before with involuntary manslaughter in connection with shooting by children, the cases generally involve very young children who pick up a parent’s unsecured gun and accidentally kill someone.

Ms. McDonald said she was not aware of such a case against the parents of a mass shooter, even though guns used in school shootings frequently come from family members.

Ms. McDonald, a Democrat, said she has received support for her decision even from Republican gun owners, some of whom have written her letters.

“Lots of gun owners say, ‘Absolutely — if you’re not going to store your weapon safely, if you’re not going to be responsible with it, you should be held accountable,’” she said. “I think this just goes so far beyond politics, if we are parents.”

Ms. McDonald said she is not thinking about higher office, but there has already been some criticism of her televised appearances.

Michael Bouchard, the Oakland County sheriff, suggested that Ms. McDonald acted prematurely in publicizing the charges against the Crumbleys — before his office was even notified — giving the parents time to flee. The F.B.I. and U.S. Marshals fugitive team were called in and apprehended the couple early the next morning in a Detroit warehouse.

Ms. McDonald, who said she gave the sheriff’s office adequate warning of the charges, said that she has patched things up with Mr. Bouchard, the only Republican holding a major elective office in Oakland County, where Democrats have dominated in recent years.

In a statement on Friday, Mr. Bouchard cited a “disconnect” between senior prosecutors and investigators that has been resolved. But Rocky Raczkowski, the Oakland County Republican chairman, said that by announcing charges against the parents before their arrest, Ms. McDonald had created confusion.

“It’s becoming a soap opera, parents’ flight risk,” Mr. Raczkowski said, “instead of run properly like a well-oiled machine. It took away from the story of the grief, it cost money, it took away police resources, but also most important, it took away from the focus, what went wrong, what went right.”

A Michigan native and former schoolteacher who became a lawyer and then an assistant prosecutor before being elected as a judge presiding over juvenile cases, Ms. McDonald now heads up an office of more than 150 employees — all wearing black lapel ribbons in honor of the dead and injured.

As a parent, Ms. McDonald said she takes gun violence personally. She and her husband of 11 years, Jeffrey Weiss, a lawyer who prefers to remain behind the scenes, have a blended family of five children. The youngest is a sophomore in college.

Her decisions in the Crumbley case suggest a tough approach to law enforcement, but during her campaign for prosecutor, she emphasized prosecutorial reform, riding the progressive wave after the murder of George Floyd in Minneapolis. Her list of endorsers included Senator Bernie Sanders of Vermont.

After defeating a longtime Democratic incumbent, Jessica Cooper, by a wide margin, and taking office in January, Ms. McDonald had spent much of the year implementing changes in the office, including embracing alternatives to incarceration and beginning data collection to detect racial disparities in prosecution.

And though she charged Ethan Crumbley as an adult, she had pushed in her campaign for treating “children like children,” holding them “accountable without unnecessary incarceration.” In office, she reviewed 27 cases of juveniles sentenced to life without parole and gave 22 of them the chance to be resentenced, leaving them eventually eligible for parole.

Perhaps her most highly publicized undertaking, until the recent carnage, involved an internal review of a 20-year-old arson case in which five children were killed.

Ms. McDonald reopened the case at the urging of the University of Michigan Law School’s Innocence Clinic.

Imran J. Syed, co-director of the clinic, said his office had been working to prove the defendant’s innocence for more than a decade but that Ms. McDonald’s predecessor had not been interested in reviewing the circumstances.

Ms. McDonald “discovered a mountain of evidence of internal misconduct,” Mr. Syed said.

Her review found that jailhouse informants who testified against the defendant were given significant benefits in return — a fact that was not disclosed to defense lawyers at the time. Her office also uncovered a video of a child witness implicating another person in the crime, evidence that also had not been revealed.

As a result of the work, Juwan Deering, 50, was released this year after 15 years behind bars. Ms. McDonald is planning a review of other old prosecutions, she said.

Criminal justice officials must abandon a “win at all cost” mentality, she said, adding, “It’s difficult, but it’s still the right thing to do.”

Giulia Heyward contributed reporting.