And unlike several other members of the Trump cabinet, Mr. Sessions has not sullied the administration with headlines over first-class jet travel, exorbitant office furnishings, lobbyist-furnished housing — or all of the above. When he is in Washington, Mr. Sessions has a turkey sandwich from the Justice Department cafeteria (base price: $5.29) for lunch, which he eats at his desk. When his team works late, he hands out granola bars, which his wife buys in bulk at Costco.
When meeting with the police, border patrol officers and the victims of crime, Mr. Sessions gives Mr. Trump all of the credit for a law-and-order crackdown he has put into effect with a speed that thrills supporters and appalls critics. With Mr. Trump, he has said, Americans are getting “support for law enforcement like we haven’t seen in many years.” He leaves out the fact that Mr. Trump’s support does not extend to him.
A Republican former senator from Alabama, Mr. Sessions was an early, fervent backer of Mr. Trump. But as attorney general, Mr. Sessions said it was his legal duty to protect the Russia investigation, even if his boss would prefer that Mr. Sessions protect him. The president’s firing of James B. Comey, the F.B.I. director who was in charge of the Russia investigation, led to Robert S. Mueller III’s appointment as special counsel. But Mr. Trump blames Mr. Sessions.
Mr. Sessions’s staff declined to make him available for an interview for this article.
Mr. Sessions laid out his philosophy for the sheriffs: “The United States of America is not an idea — it’s a nation-state. We have a Constitution, we have laws, we have borders,” and it is his job to help protect them.
Mr. Sessions has lost no time. As the opioid crisis continues to spread, he has reversed the Obama Justice Department’s “smart on crime” initiative that encouraged prosecutors to seek lighter penalties for nonviolent drug offenders. Instead, Mr. Sessions has instructed prosecutors to seek the harshest possible sentences. He opposes criminal justice overhaul efforts championed by Jared Kushner, the Koch brothers and Republican former colleagues, including Senator Charles E. Grassley, Republican of Iowa, who has defended Mr. Sessions against Mr. Trump’s attacks. Like Mr. Trump, Mr. Sessions favors the death penalty for certain drug kingpins.
Mr. Sessions has withdrawn Obama administration guidance, known as the Cole memo, discouraging prosecutors from enforcing federal marijuana laws in states that have legalized it. That drew him into conflict with Senator Cory Gardner, Republican of Colorado, which legalized recreational marijuana in 2014. Mr. Gardner blocked confirmation votes for some 20 Justice Department appointees for months, backing down only after a phone call from Mr. Trump this month, assuring him that Colorado’s marijuana industry would not be targeted.
It was Mr. Sessions who announced Mr. Trump’s decision to end the Deferred Action for Childhood Arrivals program, an Obama executive order shielding from deportation immigrants brought to the United States illegally as children. He is suing the state of California over its sanctuary laws: “It cannot be, as it seems to be today, that someone who illegally crosses the border here on a Monday and ends up in San Francisco on Wednesday can never be deported,” Mr. Sessions said in Las Cruces. “Even if they were hauling dope to San Francisco and they got arrested, they can’t be deported. Now, how illogical and insane, really, is that?”
Unaccompanied children from Central America are flooding across the border, he warned, exploiting loopholes in the law so that they “are not able to be returned quickly,” Mr. Sessions said, citing arrests of juveniles accused of smuggling drugs. “I have ordered each United States attorney’s office along the southwest border to have a zero-tolerance policy toward illegal entry. Our goal is to prosecute every case that is brought to us. There must be consequences for illegal actions.”
Todd A. Cox, director of policy at the NAACP Legal Defense and Educational Fund Inc., said Mr. Sessions was ably carrying out Mr. Trump’s plans. “He’s put in place policies that will return us to the discredited war on drugs, which had catastrophic impacts on communities of color,” Mr. Cox said in an interview. “He is doing precisely what this administration has pledged to do.”
Some Justice employees, however, see Mr. Sessions’s stoicism as capitulation. In a survey last year of employee morale conducted by the United States Office of Personnel Management and compiled by the Partnership for Public Service, the Justice Department was ranked 11th among 18 large agencies, down from sixth in 2016. Rod J. Rosenstein, the deputy attorney general, who supervises Mr. Mueller, is also fighting for his job, pressed by Mr. Trump’s congressional allies to produce hundreds of thousands of documents related to politically delicate F.B.I. investigations.
In Las Cruces, Mr. Sessions ended his speech to the sheriffs with a favored line: “We have your back. You have our thanks.”
There is no such comfort for Mr. Sessions. Congress cannot stop Mr. Trump from firing him, though Republican leaders have warned that Mr. Trump would pay a heavy political price if he does. Potential replacements for Mr. Sessions are, in the meantime, circling, including Rudolph W. Giuliani, who joined Mr. Trump’s legal team last week.
Mr. Sessions’s allies say he is not distracted. Though Mr. Trump now regrets it, he gave Mr. Sessions the law enforcement job of a lifetime, and Mr. Sessions remains focused on serving — and ignoring — the president for as long as he can.