Agriculture Secretary Tom Vilsack has left his post a week early on Friday, with no immediate replacement in sight.

Vilsack, who was confirmed on Jan. 20, 2009 and has served longer than any other member of President Obama’s Cabinet, informed department employees Friday he was stepping down. President-elect Trump has not yet nominated an agriculture secretary, despite outperforming Democratic rival Hillary Clinton in rural areas.

Farm and Foreign Agriculture Under Secretary Michael Scuse will serve as acting Agriculture Secretary, according to department spokesman Brian Mabry.

In an open letter to employees, Vilsack wrote that he wanted “to express my profound gratitude to the people who work at USDA.”

“Your work allows America to have the most productive farmers, ranchers, and producers in the world. Your work protects our families from unsafe food and our homes from dangerous forest fires,” he wrote. “Your work ensures that struggling families have enough to eat and our school children have more nutritious meals and snacks.”

crop_90VILSACK0271474402277 Tom Vilsack (Matt McClain, The Post)

The former Iowa governor, who was a finalist in Clinton’s search for a running mate last summer, did not provide an explanation for his early departure. But an individual close to him, who asked not to be identified in order to discuss a personnel matter, said Vilsack had planned the move for some time.

The Associated Press first reported Vilsack’s departure Friday evening.

Vilsack had threatened to resign in late 2015, telling Obama he did not have enough to do. In response, the president charged him with overseeing the administration’s response to the opioid and prescription drug use crisis that has ravaged many rural communities.

While many top administration officials praised Vilsack for his focus on addiction — his own mother was an alcoholic who managed to become sober toward the end of her life — as well as his work on agricultural development, climate change and children’s issues, he criticized Democrats and Republicans at times.

Publicly and privately, he complained that official Washington — including the media — did not devote sufficient attention to the men and women living outside major cities and the suburbs. “I just sometimes think rural America is a forgotten place,” he said in an interview with The Washington Post last year.

After the Democrats lost the White House in November and failed to take back the Senate, Vilsack appealed to Vice President Biden to refocus the party’s attention on the heartland.

“We need to speak more directly to our folks in rural America,” Vilsack remembered telling Biden in a subsequent Post interview. “And we have to spend time there.”

Vilsack, who was elected governor of Iowa in 1998 and presided over a Democratic surge there, has witnessed his party’s advantage erode dramatically in recent years. At this point Republicans dominate the state’s government and control both Senate seats as well as three of the four House seats.

Michigan Sen. Debbie Stabenow, the top Democrat on the Senate Agriculture, Nutrition and Forestry Committee, said in an interview Saturday that she was less concerned about the timing of Vilsack’s departure than the fact that Trump has not selected someone to succeed him after Inauguration Day.

“There’s nobody who has been a better Secretary of Agriculture than Tom Vilsack, period,” she said. “He’s put his heart and soul into it, and felt this is the moment that he should leave. What I am concerned about it is that there’s no nominee yet.”

“Agriculture is a huge part of the economy,” Stabenow said, adding that in her home state one out of every four jobs are connected to the food industry. “The fact that a major Cabinet position is not been filled yet is a real concern to me.”