WASHINGTON (AP) — President Donald Trump defended his tough trade negotiations with China, Canada and Mexico on Monday, saying that U.S. farmers have been treated "unfairly." The president vowed to turn the tide and break down large trade barriers with top trading partners.

Trump opened the week with a series of tweets pointing to his ongoing trade talks with China and a slew of U.S. allies, pointing to his work to help the nation's agriculture industry. He tweeted that farmers hadn't been "doing well for 15 years," blaming the policies of Mexico, Canada and China.

"By the time I finish trade talks, that will change. Big trade barriers against U.S. farmers, and other businesses, will finally be broken. Massive trade deficits no longer!" Trump tweeted. In a separate tweet, he wrote that China "already charges a tax of 16% on soybeans. Canada has all sorts of trade barriers on our Agricultural products. Not acceptable!"

The president last week imposed tariffs on steel and aluminum imports from top U.S. trading partners, including Canada, Mexico and the European Union. And he has threatened tariffs on up to $200 billion in Chinese imports, raising the potential for retaliation in a dispute involving the globe's two largest economies.

Senate Republicans have warned that the tariffs could dampen the economic gains from the GOP tax cuts and sour the mood among voters as lawmakers are campaigning to protect the Republican majority in Congress.

Many farm-belt lawmakers worry that the trade disputes could make American farmers the target of retaliation. Mexico, for example, has said it will penalize U.S. imports including pork, apples, grapes and cheeses.

Groups backed by the influential Koch brothers' network announced they were launching a new multimillion dollar campaign to oppose tariffs and highlight the benefits of free trade. The groups — Freedom Partners Chamber of Commerce, Americans for Prosperity and The LIBRE Initiative — called on Congress to exert oversight by requiring House and Senate votes on any new tariffs and urged the lifting of the recent steel and aluminum tariffs as well as those being proposed on goods from China.

Freedom Partners' James Davis said they wanted to confront "protectionist ideas" they say would hold back the economy.

Trump has also received blowback from world leaders. British Prime Minister Theresa May told Trump that his decision to impose tariffs on steel and aluminum imports from the European Union was "unjustified and deeply disappointing."

Trump's hard-line rhetoric comes as Commerce Secretary Wilbur Ross returns from China as part of the ongoing trade discussions. The White House said the meeting focused on reducing the U.S. trade deficit by having China buy more agricultural and energy products.

But China said any agreements with Washington in their talks on settling a sprawling trade dispute "will not take effect" if threatened U.S. sanctions including tariff hikes go ahead.

__

AP Congressional Correspondent Lisa Mascaro contributed to this report.