By Aruna Viswanatha and Jeff Mason

WASHINGTON (Reuters) - President Barack Obama on Saturday picked Brooklyn federal prosecutor Loretta Lynch to be the next U.S. attorney general, signaling a change in style but a continuation of the law enforcement priorities the administration has pursued.

If confirmed, Lynch, 55, would be the first black woman to serve in the post, bringing with her a family history that stretches back generations to great-great-grandparents who were slaves.

Obama said he hoped the Senate would not delay in confirming Lynch, who he said had distinguished herself as tough and fair in her two stints as the U.S. attorney in the Eastern District of New York.

Lynch would replace Eric Holder, the first black attorney general, who has held the job since Obama took office in 2009. She was among several candidates Holder had recommended to succeed him.

Obama's selection of Lynch is a departure from his tendency to put candidates with whom he has a long personal history into top jobs. It is the first major personnel change he has announced since Republicans won control of the Senate in congressional elections on Tuesday. Holder said in September that he planned to step down.

Senate Republican leader Mitch McConnell, who is slated to become majority leader, said Lynch should be considered by the new Republican-controlled body next year.

"Ms. Lynch will receive fair consideration by the Senate. And her nomination should be considered in the new Congress through regular order," he said in a statement.

Holder, one of Obama's closest allies, has had a rocky tenure as attorney general. He clashed frequently with congressional Republicans over gun control, same-sex marriage, and a desire to try terrorism suspects in civilian rather than military courts.

In one 2011 email released earlier this week, Holder referred to Republican members of the House Oversight Committee chaired by Darrell Issa as "Issa and his idiot cronies".

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LITTLE CONTROVERSY

Lynch, a Harvard-trained lawyer from North Carolina, has stirred little controversy during her decades as a prosecutor.

"I've worked with her on and off for almost 25 years, and I've never seen her lose her temper," said Alan Vinegrad, who worked as Lynch's chief assistant and is now a lawyer at Covington & Burling.

The next attorney general will face many challenges, including managing counter-terror initiatives aimed at Islamic State militants, balancing privacy rights against government surveillance efforts, and deciding whether to bring charges in connection with the shooting death of an unarmed black teenager by a white police officer in Ferguson, Missouri.

Lynch's deep experience in civil rights, terrorism and corporate fraud cases, and her work leading a committee that advises the attorney general on policy issues, could result in continuity with Holder.

Holder has built much of his legacy on trying more terrorism suspects in federal courts, and on civil rights issues, as he has pushed to enforce civil and voting rights laws and to reduce sentences for low-level drug offenders.

Since the attacks of Sept. 11, 2001, Lynch's office handled more terrorism prosecutions than most other offices in the country, including the prosecution of three men, two of whom plead guilty and a third who was convicted at trial, in connection with a plot to stage suicide attacks on the New York subways in 2009. Her office also prosecuted another man who pleaded guilty in 2013 to trying to bomb the New York Federal Reserve.

"Loretta might be the only lawyer in America who battles mobsters and drug lords and terrorists and still has the reputation for being a charming 'people person,'" Obama said in announcing the nomination.

Lynch has also spoken of her family of preachers, including her grandfather, who hid black people unjustly pursued by a local sheriff under his floorboards. Her great-great grandfather, she has said, was a free black man in North Carolina who re-entered bondage in order to marry her great-great grandmother, who was enslaved.

"I will work every day to safeguard our citizens, our liberties, our rights, and this great nation which have given so much to me and my family," said Lynch, whose married name is Hargrove.

(Reporting by Aruna Viswanatha and Jeff Mason and Jessica Dye; Editing by Bill Trott and Stephen Powell)