“We can’t wait” was Education Secretary Arne Duncan’s frequent refrain. But as he leaves his post this December, his forceful strategy to push dramatic changes to the U.S education system is being tested.

The aggressiveness and urgency that defined his efforts to transform American schools alienated friends and could, in the end, be what derails his reforms.

During his tenure, one of the longest in President Barack Obama’s cabinet, Duncan made a deep mark on U.S. schools with a series of major efforts stretching from early education to college. The administration promised $1 billion in new spending on preschool; spurred states to adopt controversial K-12 reforms such as performance-based teacher evaluations and the adoption of the Common Core State Standards through its Race to the Top grant program and waivers to the No Child Left Behind law; significantly expanded the federal School Improvement Grant program to turn around low-performing schools; targeted for-profit colleges and attempted to increase accountability in the higher education sector; and pushed a proposal by the president to make community college free.

But Duncan’s resignation comes as Congress is deliberating over reauthorization of the No Child Left Behind law and considering rewrites that would limit the ability of the education department to get involved in state policy, leaving many wondering whether Duncan’s seven years of intense reforms will stick. Politico reported that “just this week, Duncan said he thought the forthcoming resignation of House Speaker John Boehner would make it more difficult to get the law updated.”

“I can only think that our odds of having it pass now are worse, not better, which is really disappointing,” he was quoted as saying.

He’s also leaving as a bipartisan Senate bill and a Republican-backed House bill seek to limit the federal government’s role in public education and to take aim at policies Duncan pushed, such as teacher evaluations based partly on test scores and aggressive school turnaround strategies.

At the same time, many states are facing growing backlash over the increased emphasis on standardized testing and are slowing down plans to revamp teacher evaluation systems or retreating on Common Core (although some have adopted near replicas to replace it). And the data on the effectiveness of some strategies, including the School Improvement Grant program and new teacher evaluation policies, has been mixed.

Nevertheless, his supporters are praising him for his sense of urgency and forceful tactics. Purdue University President Mitch Daniels, the former Indiana governor, said he regretted deeply that Duncan is leaving.

“He has placed America’s children and their academic success at the center of his work every day of his tenure, often challenging the most intransigent and powerful special interests in our political system to do so,” Daniels wrote in a statement. “Those who succeed him in this administration and the next would be wise to emulate and extend his example.”

In 2009, Duncan and his department were given access to $100 billion for education in the American Recovery and Reinvestment Act. Often bypassing Congress, Duncan used the money to work directly with states, persuading them to adopt favored policies by providing incentives through Race to the Top, a $4.35 billion competitive grant program in which states were awarded points for adopting ideas such as performance-based teacher and principal evaluations, higher academic standards, and raising charter school caps. Two years later, the administration gave states the opportunity to apply for waivers that exempted them from a federal requirement under No Child Left Behind that 100 percent of American students be proficient in reading and math by 2014.

Most states bowed to the pressure. For example, 44 states and the District of Columbia adopted the Common Core, the “college and career-ready” standards created by states but promoted by Duncan.

But as new policies began rolling out, a backlash grew. Progressives protested the increased emphasis on tests. Conservatives balked at the administration’s role in promoting the Common Core standards, saying it was federal overreach that undermined state and local control of public education. And even supporters worried that tying teacher evaluations to student outcomes while rolling out difficult new standards was ill conceived.

Most notably the national teachers unions — a significant force in the Democratic Party — turned on him. But other allies and even some critics say his reforms will last. “Arne Duncan was one of the president’s best appointments. He has a big heart, cares about children, and I have enjoyed working with him,” Senate education committee Chairman Lamar Alexander (R-Tenn.), one of Duncan’s occasional adversaries, said in a statement. “When we disagree, it is usually because he believes the path to effective teaching, higher standards, and real accountability is through Washington, D.C., and I believe it should be in the hands of states, communities, parents and classroom teachers.”

Nina Rees, president and CEO of the National Alliance for Public Charter Schools, applauded Duncan for his support of charter schools.

“His leadership on behalf of the federal Charter Schools Program has enabled the dramatic growth in the number of high quality charter schools, ensuring that hundreds of thousands more students now have access to better schools regardless of their family income or zip code,” she wrote in a statement.

Early education advocates were also grateful for the administration’s elevation of early education. “Secretary Duncan’s leadership and unwavering dedication to early childhood education has made an immeasurable difference in the lives of countless young learners,” First Five Years Fund executive director Kris Perry said in a statement. “He has harnessed what all of the research shows about the benefits of investing in early learning, and successfully incorporated it into the everyday mission and policy goals of the Department of Education. We are incredibly grateful to Secretary Duncan for being a champion of American’s greatest resource – its children.”

In higher education, Duncan pushed for income-based repayment of student loans, and recently claimed some success as default rates decreased.

Duncan also sought to protect Americans from for-profit colleges accused of taking advantage of federal funds, with proposed regulations that would stop the flow of funds to low-performing schools whose graduates don’t earn enough to repay their loans.

“Higher education should open up doors of opportunity, but students in these low-performing programs often end up worse off than before they enrolled: saddled by debt and with few—if any—options for a career,” Duncan said in a statement last year.

President Obama has appointed John King Jr., a senior official in the education department and previously New York’s education commissioner who oversaw the roll out of Common Core there, to serve as interim education secretary for the duration of Obama’s presidency. King “is no stranger to controversy,” as Alyson Klein of Education Week put it, and will most certainly follow Duncan’s playbook.

Duncan had been widely expected to stay until the end of Obama’s second term and the president expressed his “regret and sorrow” that his friend and basketball partner is leaving. “I’ll be honest. I pushed Arne to stay,” he said in a televised press conference.

Duncan plans to return to Chicago to spend time with his family, according to The Associated Press, which was the first to report his plans to resign. He has not decided what his next move will be, but said he hopes to continue increasing opportunities for children in some capacity.

This story was produced by The Hechinger Report, a nonprofit, independent news organization focused on inequality and innovation in education.