BEIJING—China’s leadership pledged to ensure economic stability as its trade fight with the U.S. started to pinch growth, signaling that a bigger stimulus could be on the horizon.

A Tuesday meeting of the Politburo, whose members are China’s top arbiters of power, highlighted the external challenges faced by the world’s second-largest economy. Without specifically mentioning the trade conflict with the U.S., a statement issued after the meeting by state media made clear the brawl is a big threat to growth and stability, trumping...