(AP) -- Arne Duncan, who followed President Barack Obama to Washington to serve as his education secretary, announced Friday he will step down following a seven-year tenure marked by a willingness to plunge head-on into the heated debate about the government's role in education.

Sidestepping a confirmation fight in Congress, Obama tapped John King Jr., a senior Education Department bureaucrat, to run the department while leaving the role of secretary vacant for the remainder of his presidency.

One of Obama's longest-serving Cabinet members, Duncan is among the few to form a close personal relationship with the president. After his departure in December, Agriculture Secretary Tom Vilsack will be the sole member of Obama's Cabinet still in his original role.

"Arne's done more to bring our educational system, sometimes kicking and screaming, into the 21st century than anybody else," Obama said, praising Duncan at the White House as one of the most consequential secretaries in the department's history.

Duncan, who plans to return to Chicago to join his family, choked up as he reflected on his run in Washington and his roots as the child of Chicago teachers.

"All our life we saw what kids could do when they were given a chance. That's why we do this work today," Duncan said.

In an unconventional move, Obama asked King to oversee the Education Department, but declined to nominate him to be secretary, which would require confirmation by the Republican-run Senate. Elevating King in an acting capacity spares Obama a potential clash with Senate Republicans over his education policies as his term draws to a close.

"We do not intend to nominate another candidate," said a White House official who wasn't authorized to comment by name and spoke on condition of anonymity. Republicans pointed out that Obama has previously complained that acting secretaries cannot fulfill all the duties of Senate-confirmed agency heads.

Duncan's tenure coincided with a roiling debate about perceived federal overreach into schools that remains a potent issue as he leaves office. Navigating a delicate divide, Duncan sought to use the federal government's leverage to entice states to follow Washington's preferred approach to higher standards, prompting resistance from all sides.

On the right, Republicans and state leaders accused Duncan of a heavy-handed federal approach that sidestepped lawmakers and enforced top-down policies on local schools. Critics blasted the department for linking federal dollars to state adoption of standards such as the Common Core, a controversial set of curriculum guidelines. His signature initiative was Race to the Top, in which states competed for federal grants, with strings attached.

On the left, Duncan clashed over policy with teachers' unions -- including the largest, the 275,000-member National Education Association, which once called on Duncan to resign. Traditionally reliable Democratic allies, labor leaders bristled at his strong support for charter schools and the use of student test scores to evaluate teachers.

Throughout his tenure, Duncan stood firmly behind federal standardized testing requirements, even as he readily handed out waivers exempting states from George W. Bush-era requirements under No Child Left Behind. Duncan cast the federal testing as a civil rights issue, critical to making school ensure that students of all races and backgrounds succeed. The Education Department pointed to statistics showing the high school graduation rate under Duncan hit a new high of 81 percent.

Occasionally flashing impatience with criticism, Duncan raised eyebrows in 2014 when he cast opponents as "white suburban moms who -- all of a sudden -- their child isn't as brilliant as they thought they were." He later said he regretted the "clumsy phrasing."

Part of the Chicago cohort that converged on Washington after Obama's election, Duncan previously ran the Chicago public school system, although he never worked as a teacher. A basketball player who played professionally in Australia, Duncan was once a regular in Obama's weekend games.

"Arne Duncan was one of the president's best appointments," said Sen. Lamar Alexander, R-Tenn., who frequently clashed with Duncan as chairman of the Senate's education panel. He added that they disagreed on the issue of federal versus local control of schools.

In his previous role at the Education Department, King oversaw preschool-through-high school education and managed the department's operations. He served earlier as state education commissioner in New York, where he drew many of the same critiques from teachers unions that Duncan did in Washington.

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