CASTAIC, Calif. — When Kamala D. Harris, a Democrat, was the newly elected district attorney of San Francisco in 2004, she walked into a firestorm after deciding not to pursue the death penalty for a man accused in the killing of a police officer — drawing attacks from law enforcement leaders and even Senator Dianne Feinstein, one of the most respected Democrats in the state.

Six years later, when Ms. Harris ran for state attorney general, national Republicans poured more than $1 million into the race, trying to defeat her with charged advertisements invoking the death penalty case. Ms. Harris barely defeated her Republican opponent, the district attorney of Los Angeles.

But now, at age 50 and after winning a second term, Ms. Harris has suddenly established herself as the dominant candidate in the race to replace Senator Barbara Boxer, a Democrat who announced in January that she would retire next year. With a speed and efficiency that startled many in her party, Ms. Harris has appeared, at least for now, to dispatch what most people had expected would be a sprawling generational battle with powerful ethnic overtones, given that Latinos now make up nearly 40 percent of California’s population.

She herself would be a pioneering figure, if elected — simultaneously the first black and the first South Asian senator from California.

Within days of Ms. Boxer’s announcement, Ms. Harris declared she was in, and followed that up with a day-by-day machine-gun patter of high-profile endorsements. Among them were Senator Elizabeth Warren of Massachusetts; leading members of California’s congressional delegation and State Legislature; and Emily’s List, a political fund-raising organization that backs female Democrats who support abortion rights.

That had the intended effect of helping to clear the field of some of her biggest potential challengers, including Antonio R. Villaraigosa, the former Los Angeles mayor, and Tom Steyer, the billionaire environmental activist. Lt. Gov. Gavin Newsom decided to run for governor instead, in 2018.

In getting to this point, Ms. Harris — her first name is pronounced KOM-ala — displayed what many associates described as a relentless political style that has marked her career, starting with her first election as the San Francisco district attorney. With an eye to what happened in 2004, she went out of her way to line up and highlight the support of some of this state’s top law enforcement associations as part of her endorsement rollout after she announced her Senate candidacy.

“I always start my campaigns early, and I run hard,” Ms. Harris said in an hourlong interview. “Maybe it comes from the rough-and-tumble world of San Francisco politics, where it’s not even a contact sport — it’s a blood sport. This is how I am as a candidate. This is how I run campaigns.”

“And she always wins,” Sean Clegg, an aide seated at her side, added in a stage whisper.

“Oh God, don’t say that,” Ms. Harris said, knocking the wooden table before her with three sharp, superstitious raps. “Don’t, don’t, don’t! So far, so good.”

For all that, there is no assurance of easy sailing in an election still a year and a half away. Running a district attorney’s office, or being a state attorney general, means having responsibility for many law enforcement decisions and cases that could become political fodder.

This week, for example, Ms. Harris had to weigh in on a proposed ballot measure that would mandate the killing of sexually active gay people by “bullets to the head” — or by “any other convenient method.” Such voter initiatives go to the attorney general to process, and the law appears to give Ms. Harris little discretion but to do so. But she took the highly unusual step of asking a court to step in and block the measure.

Ms. Harris will probably face a Democratic challenger before the June 2016 open primary, albeit not necessarily one with the political resources of the figures who stepped aside. Three members of Congress — Adam B. Schiff, Loretta Sanchez and Xavier Becerra — are said to be considering races. Ms. Sanchez and Mr. Becerra could appeal to a constituency hoping that this might be a time for California to elect its first Latino senator.

“Politics abhors a vacuum,” said Bill Carrick, a Democratic consultant who advises Ms. Feinstein. “Somebody is going to fall into this.”

On the Republican side, Rocky Chávez, a former Marine colonel and two-term member of the State Assembly, who is also Hispanic, said he would run. “A lot of things can happen on the way to a coronation,” he said in an interview.

Ms. Harris’s mother was from India and her father of Jamaican descent. In one politically awkward moment at a fund-raiser in 2013, President Obama called her “by far the best-looking attorney general in the country.”

Along with Mr. Newsom, Mr. Villaraigosa and Eric Garcetti, the mayor of Los Angeles, Ms. Harris has long been viewed as at the vanguard of the next generation of leaders in a state whose political hierarchy has been dominated by Democrats over 70. Gov. Jerry Brown is in his fourth term in office. Ms. Boxer was elected to the Senate in 1992.

Ms. Harris’s elevated political status was on display here this month during what should have been a relatively sleepy stop on her schedule: the Pitchess Detention Center, settled on a 2,600-acre ranch in a remote corner of Los Angeles County.

It was bustling with elected officials, social service workers, prison guards, inmates, the county sheriff — and a dozen television camera crews, waiting for Ms. Harris to describe, in detail, “Back on Track,” her pilot program to reduce recidivism in county jails.

Recidivism, a subject about which she wrote a book — “You can get it for $2.99 on Amazon,” Ms. Harris said — has been one issue she has focused on as attorney general. She has also advocated tougher truancy laws and joined other attorneys general in winning a settlement against United States banks for illegally foreclosing on homes.

Still, questions remain about what kind of candidate she will be. She has been involved in only one tough statewide contest, her first run for attorney general, and she came close to losing. “She is going to have to enter into an entirely new political arena,” said Gale Kaufman, a Democratic political consultant in Sacramento. “Up until now, she primarily talked about issues that either were related to her being the D.A. or the attorney general.”

At almost every turn, Ms. Harris invokes her background as a prosecutor. “I have personally prosecuted everything from low-level offenses to homicides,” she said, when discussing immigration.

Ms. Boxer and Ms. Feinstein are both known as passionate and outspoken. Ms. Harris has long been known as more cautious. “She’s more attorney than general,” said Jamie Court, the president of Consumer Watchdog, an advocacy group.

Ms. Harris, for example, said she did not have an opinion on whether the high-speed train that Mr. Brown has made a centerpiece of his administration — and which has drawn strong opposition from Republicans — should proceed. “I haven’t looked closely enough at it as an issue where I’ve formed an opinion,” she said.

“I’m a career prosecutor,” she said. “I have been trained, and my experience over decades, is to make decisions after a review of the evidence and the facts. And not to jump up with grand gestures before I’ve done that. Some might interpret that as being cautious. I would tell you that’s just responsible.”

Ms. Harris pointed to her attorney general race as evidence of her strength going into this campaign. “A lot of folks said we wouldn’t be able to win from the beginning: ‘This D.A. from San Francisco, this woman of color who is against the death penalty, running to become the top cop of the state,’” she said.

Still, Ms. Harris said, with another knock of her knuckles on the table, she was not assuming she would glide into the Senate. “We don’t know that yet, because the lady ain’t singing,” she said. “I’m sure someone is going to jump in.”