Panda diplomacy, in its current form, works like this: China loans pandas to a zoo in the United States or another country, and the zoo pays an annual fee — usually $500,000 to $1 million each — to keep the pandas for at least a few years. The animals serve as good-will ambassadors for China while, experts said, softening the country’s authoritarian image and drawing attention away from its record of human rights abuses.

“It’s soft power,” said Andrew J. Nathan, a political science professor at Columbia who specializes in Chinese politics and foreign policy.

“Pandas are very cute and lovable,” he said, “so it fits into that kind of friendship diplomacy image.”

But now, a bill in Congress is taking aim at this longstanding arrangement — specifically, the stipulation that panda cubs born abroad must be shipped to China within a few years.