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By JOSH LEDERMAN, Associated Press

WASHINGTON (AP) — Education Secretary Arne Duncan, one of the longest-serving members of President Barack Obama's Cabinet, will step down in December, officials said Friday.

In a letter to his staff obtained by The Associated Press, Duncan said he's returning to Chicago to live with his family. He said he isn't sure what he will do next, but that he hopes his future will "continue to involve the work of expanding opportunity for children."

President Barack Obama has tapped John King Jr., a senior official at the Education Department, to run the department for the remainder of his administration. Obama doesn't intend to nominate King or to nominate another education secretary during the rest of his presidency, but will instead ask King to serve in an acting capacity, said a White House official, who wasn't authorized to comment by name and requested anonymity.

That approach will spare Obama a confirmation fight over a nominee in the Senate.

Duncan is one of just a few remaining members of Obama's original cabinet. Agriculture Secretary Tom Vilsack and Office of Management and Budget director Shaun Donovan have also served in the Cabinet since the first term. Donovan, however, first served as secretary of Housing and Urban Development.

Duncan came to Washington from Chicago, where he ran Chicago's public school system. As part of the Chicago cohort that followed Obama to Washington, Duncan is one of few Cabinet members who has a personal relationship with the president. A basketball player at Harvard University who played professionally in Australia, Duncan was once a regular in Obama's weekend basketball games.

As secretary, Duncan prioritized K-12 education and made his first signature initiative the Race to the Top program, in which states competed for federal grants. The program became a flashpoint in the fight over federal involvement in education. Critics argued it encouraged states to adopt the Common Core, a controversial set of curriculum guidelines that become symbolic of federal overreach.

Duncan showed little patience for criticism of the program and the standards. In 2014, he cast critics as "white suburban moms who — all of a sudden — their child isn't as brilliant as they thought they were and their school isn't quite as good as they thought they were, and that's pretty scary." Duncan later said he regretted the "clumsy phrasing."

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Associated Press writer Kathleen Hennessey contributed to this report.

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Reach Josh Lederman at http://twitter.com/joshledermanAP