WASHINGTON — Jefferson Beauregard Sessions III, as the courtly senator from Alabama used to be known, was a stalwart Justice Department prosecutor for almost 15 years, a job he called the adventure of a lifetime.
Today, Mr. Sessions has a growing list of gripes about how the Obama administration has run his old department, from its “breathtaking” stance on immigration to its “shameful” refusal to defend a federal ban on gay marriage.
“I’m not happy about what’s happened to my Department of Justice,” Mr. Sessions said last year, jabbing his reading glasses in the air at the Senate confirmation hearing for Attorney General Loretta E. Lynch.
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After nearly a quarter-century away, Mr. Sessions — now known simply as Jeff — is poised to return to the department to clean house as President-elect Donald J. Trump’s nominee for attorney general, with a mandate to carry out the “law and order” agenda Mr. Trump promised on the campaign trail.
If he is confirmed, Mr. Sessions, who is considered one of the most conservative members of the Senate, will most likely push for wholesale changes and hard-line stances on immigration, terrorism, crime, drugs and guns. Democrats fear he could wipe away progress in civil rights, changes in sentencing and police accountability.
“The Justice Department is likely to be one of the most transformed departments in the cabinet in a Trump administration, and with an Attorney General Sessions, you’d obviously see a very strong law-and-order figure at the top,” said Jonathan Turley, a George Washington University law professor.
“Much of his self-identity is as a prosecutor — a real, in-the-trenches prosecutor,” said Mr. Turley, who testified before Mr. Sessions at a Senate hearing last year about the Obama administration’s use of executive authority.
Mr. Sessions, 69, was the first senator to endorse Mr. Trump in February, when many Republicans were still shunning the businessman. He has since become a close adviser.
The Senate Judiciary Committee, where Mr. Sessions has served for years and sometimes clashed with fellow members, will consider his nomination. Democrats are eager to interrogate him on the rockier patches of his long career, particularly the accusations of racially charged comments in the 1980s that derailed his nomination as a federal judge.
Critics charge that the controversy was a harbinger of hostility toward minorities that has continued in his two decades as a senator.
“He’s one of the most strident anti-immigrant, anti-Muslim, anti-L.G.B.T. voices in the Senate,” said Marge Baker, executive vice president of People for the American Way, a liberal civil rights group in Washington.
Mr. Sessions, like Mr. Trump, has made tougher immigration policies a central priority. He has said President Obama’s Justice Department flouted the will of Congress by failing to deport millions of undocumented immigrants.
When many Republicans distanced themselves from Mr. Trump’s startling campaign proposal to ban Muslim immigration, Mr. Sessions said he was open to considering it in a Breitbart interview last December with Stephen K. Bannon, who was chosen by Mr. Trump this week as a top White House adviser.
“At the same time, we’re in an age that’s very dangerous, and we’re seeing more and more persons enter, and a lot of them have done terrorist acts, and a lot of them believe it’s commanded by their religion,” he said. “So I think it’s appropriate to begin to discuss this.”
As the nation’s top law enforcement officer, Mr. Sessions and the political appointees under him would hold wide discretion in shaping policies across the federal government and in overseeing the enforcement of federal laws at the F.B.I. and other agencies within the Justice Department.
His department would also play an important role in advising the White House on Supreme Court nominees and working to get them confirmed in the Senate. Mr. Sessions, an avid student of constitutional history, has called the Justice Antonin Scalia, who died in February, a model, hailing him for “an unwavering commitment to the rule of law” rather than judicial activism.
Mr. Sessions was born in Selma, Ala., the son of a country store owner. The diminutive Mr. Sessions was an Eagle Scout known as “Buddy” as a child. His parents, he said, instilled in him the values of “honesty, hard work, belief in God and parental respect.”
With a law degree from the University of Alabama, Mr. Sessions worked as a private attorney and as an assistant federal prosecutor before President Reagan tapped him in 1981 as the United States attorney for the southern district of Alabama. He held that post for 12 years.
Accusations of inappropriate racial remarks — one black prosecutor said Mr. Sessions called him “boy,” and another prosecutor said he referred to groups like the N.A.A.C.P. and the American Civil Liberties Union as “un-American” — doomed his nomination for a lifetime appointment to the federal bench in 1986.
But Mr. Sessions took satisfaction in not only winning election to the Senate a decade later, but also in earning a seat on the powerful judiciary committee, the same panel that had rejected his nomination as a judge.