President Trump moved Thursday to require more food stamp recipients to find jobs, saying government aid should not become “a way of life.”

On Mr. Trump’s orders, the Agriculture Department is proposing a rule that would require more “able-bodied recipients” of food stamps to work at least 20 hours per week. The move seeks to limit states’ ability to waive the program’s work requirements.

The president took the step as he signed into law the $400 billion farm bill, in which lawmakers failed to include the food-stamp proposal. Mr. Trump said the strong economy has lifted 4.6 million Americans off food stamps since his 2016 election, and he wants to go further.

“Millions of able-bodied, working-age adults continue to collect food stamps without working or even looking for work,” the president said at the farm-bill signing. “Our goal is to move these Americans from dependence to independence, and into a good-paying job and rewarding career.”

He said the proposed rule would close a “loophole” in food-stamp requirements and result in stronger families.

Agriculture Secretary Sonny Perdue said long-term reliance on government aid “has never been part of the American dream.”

“As we make benefits available to those who truly need them, we must also encourage participants to take proactive steps toward self-sufficiency,” Mr. Perdue said. “Moving people to work is common-sense policy, particularly at a time when the unemployment rate is at a generational low.”

The farm bill reauthorizes agriculture and conservation programs while leaving untouched the food-stamp program, known as the Supplemental Nutrition Assistance Program, which serves about 40 million Americans. The House proposed to tighten work requirements on food stamps, but the Senate didn’t go along.

Currently, able-bodied adults ages 18-49 without children can receive food stamps for three months if they fail to work at least 20 hours per week. States can waive the requirement in regions where the unemployment rate is higher than 10 percent or where the rate is at least 20 percent higher than the national average. The jobless rate was 3.7 percent in November.

The new rule would allow states to waive the work requirement only in areas where the unemployment rate is 7 percent or higher. Governors would need to endorse waivers, which would be good for only one year instead of the current two years.

“This restores the dignity of work to a sizable segment of our population, while it’s also respectful of the taxpayers who fund the program,” Mr. Perdue said.

Democrats, who will take over the House majority next month, vowed to stop the administration’s effort. Rep. Rosa DeLauro, Connecticut Democrat, introduced legislation Thursday that would prevent Mr. Perdue from implementing the stricter work requirement, calling the proposal “immoral.”

“President Trump’s cruelty could not be clearer: Passing trillions in tax cuts for millionaires, billionaires and corporations — on top of the massive giveaways to corporate agribusinesses in the 2018 farm bill — while taking food assistance away from the unemployed,” she said. “The Trump administration is shamelessly picking up where House Republicans left off because they could not get this passed in Congress. We cannot cut off this vital lifeline by rigging SNAP against the very people who the program was created to help.”

The Agriculture Department said the proposed rule “is meant to restore the system to what it was meant to be: assistance through difficult times, not lifelong dependency.

Mr. Perdue said that ideal over time “has been watered down by out-of-control administrative flexibility in SNAP.”

“Americans are generous people who believe it is their responsibility to help their fellow citizens when they encounter a difficult stretch,” Mr. Perdue said. “That is the commitment behind SNAP. But like other federal welfare programs, it was never intended to be a way of life.”

Agriculture officials said nearly half of the able-bodied, childless adults who receive food stamps live in areas under a waiver. They said the new rule would save about $15 billion over 10 years.