WASHINGTON — President Trump’s escalating economic war with China highlights a challenge for Democrats hoping to unseat him in 2020: They’ll have a hard time being tougher on trade than he is.

For years, Democrats in Congress have been warning that China is an economic aggressor bent on undermining American industry. They have denounced the North American Free Trade Agreement for outsourcing jobs and criticized China for manipulating its currency to make Chinese products cheaper. They have vowed to use federal procurement, tariffs and other tools to help American workers.

Mr. Trump has stolen that playbook and gone further. On Monday, his administration formally designated China a currency manipulator, a step some Democrats have demanded for years. Last week, the president moved forward with plans to tax nearly every toy, laptop and sneaker that China sends to the United States. Mr. Trump has also renegotiated NAFTA, imposed tariffs on foreign metals and strengthened “buy American” rules so that federal projects use more materials from the United States.

So far, many of these efforts have not produced the kind of change Mr. Trump promised. His revised NAFTA, the United States-Mexico-Canada Agreement, is languishing in Congress, and his sweeping tariffs have prompted China and Europe to retaliate against American products, particularly farm goods. The president’s trade war with China has begun driving up costs for consumers and businesses.