SAN FRANCISCO — With President Trump vowing to get tougher on trade, troubled American makers of everything from steel tubing and aluminum foil to washing machines have lined up to ask Washington for protection from foreign rivals.

But Mr. Trump’s first big international trade fight could be over solar panels.

Major manufacturers in the United States and China, as well as numerous other businesses that buy and use solar panels, are readying for a clash that could begin as soon as January. The solar panel dispute comes at a time when senior administration officials have been signaling their intention to take a much tougher trade stance toward China, where most solar panels are made.

The solar panel industry could be Mr. Trump’s first test of whether his harsh language toward China will result in significant trade measures — and whether those moves would help restore American businesses. Factories in China now account for more than two-thirds of the world’s production, up from a negligible share a decade ago. Faced with intense competition, more than a dozen solar companies in the United States have closed factories over the past six years.

China’s push to become a major maker of solar panels has driven down global prices by close to 90 percent over the past decade, helping international efforts to curb emissions of planet-warming greenhouse gases. That has blurred the lines over the pending solar trade fight even within the United States, where American manufacturers are squaring off against American installers and users of the panels.