(AP) -- President Barack Obama has chosen New York federal prosecutor Loretta Lynch to be the next attorney general, sending a Washington outsider as a high-profile confirmation test amid a power shift in the Senate.

Obama plans to announce Lynch's nomination Saturday at a White House event that was quickly arranged after news organizations reported her selection Friday. He had planned to wait until later this month to introduce his trail-blazing choice - Lynch would be the first black woman in the job. The outgoing attorney general, Eric Holder, was the first black head of the Justice Department.

But with their hand forced, White House officials said they would leave it up to Senate leaders to work out the timeline for her confirmation, urging that it be as soon as possible. Democrats and Republicans have told the White House it would be difficult and damaging to the nominee politically to try to push her through while Democrats control of the Senate. Republicans will oversee her confirmation with the next Congress.

Lynch, 55, is the U.S. attorney for the Eastern District of New York, which covers Brooklyn, Queens, Staten Island and Long Island, a position she also held under President Bill Clinton.

"Ms. Lynch is a strong, independent prosecutor who has twice led one of the most important U.S. attorney's offices in the country," Obama press secretary Josh Earnest said in a statement.

It's unusual for Obama to pick someone he doesn't know well for such a sensitive administration post, but the White House considered her distance an asset that could help in the confirmation process. Another candidate Obama asked to consider the job, former White House counsel Kathy Ruemmler, withdrew from consideration out of concern that her close relationship to Obama could lead to a difficult confirmation battle.

Republicans have promised tough scrutiny of Obama's pick after years of battles with Holder, who is close to Lynch and appointed her as chair of a committee that advises him on policy. Holder has been an unflinching champion of civil rights in enforcing the nation's laws and his successor will be left to grapple with several prominent civil rights issues that have been elevated on his watch.

Those issue include possible federal charges in the deaths of Michael Brown in Ferguson, Missouri, and Trayvon Martin in Sanford, Florida; enforcement of the Voting Rights Act after the Supreme Court threw out a major protection; reduction of racial profiling in federal investigations; changes in how federal prosecutors negotiate sentencing; changes in the death penalty system; and efforts to reduce tensions between local police departments and minority communities.

Sen. Chuck Grassley of Iowa, the top Republican on the Judiciary Committee, expressed "every confidence that Ms. Lynch will receive a very fair, but thorough, vetting by the Judiciary Committee."

"U.S. attorneys are rarely elevated directly to this position, so I look forward to learning more about her, how she will interact with Congress, and how she proposes to lead the department," Grassley said. "I'm hopeful that her tenure, if confirmed, will restore confidence in the attorney general as a politically independent voice for the American people."

Lynch was previously best known on Capitol Hill for filing tax evasion charges against Rep. Michael Grimm, a Republican accused of hiding more than $1 million in sales and wages while running a restaurant. Grimm, who won re-election Tuesday, has pleaded not guilty and is to go to trial in February.

Lynch has overseen bank fraud and other corruption cases. She charged reputed mobster Vincent Asaro and his associates for the 36-year-old heist of $6 million in cash and jewelry from a Lufthansa Airlines vault at Kennedy Airport, dramatized in the movie "Goodfellas."

During her first tenure in the Eastern District, Lynch helped prosecute police officers who severely beat and sexually assaulted Haitian immigrant Abner Louima.

Lynch grew up in North Carolina, the daughter of a school librarian and a Baptist minister. She received undergraduate and law degrees from Harvard, where Obama graduated from law school seven years after her. Personally, she goes by Loretta Lynch Hargrove, having married Stephen Hargrove in 2007.