Republican presidential candidate Donald Trump fired back at the U.S. Chamber of Commerce on Wednesday, saying the nation's largest business association needed to "fight harder" for American workers, after it issued a scathing criticism of his economic platform.

The Washington-based lobbying group, which represents the nation's largest corporations and business interests, is typically a reliable backer of Republican policies. But it took issue on Tuesday with Trump's vocal opposition to international trade deals, calling his proposals "dangerous" ideas that would push the United States into another recession.

Trump struck back the following day, saying the organization needed to "fight harder" for American workers.

"Why would the US Chamber be upset by the fact that I want to negotiate better and stronger trade deals or that I want penalties for cheaters?" the wealthy businessman wrote on Twitter.

In speeches on Tuesday, Trump called for renegotiating or scrapping the North American Free Trade Agreement (NAFTA) with Canada and Mexico, which he called job killer, and reiterated his opposition to the Trans-Pacific Partnership among 12 Pacific Rim countries. He also singled out China's trade and currency policies for criticism.

The Chamber has consistently backed international trade deals.

The public disagreement between the presumptive Republican nominee and the business group was unusual, one of a series of reminders that Trump still struggles with uniting his party. The Republicans and many business leaders tend to share policy goals and work in lockstep. Many business leaders have also traditionally been big donors to Republican candidates.

But fighting against trade deals has proven successful for Trump among voters concerned about the loss of manufacturing jobs.

Peter Navarro, a Trump trade policy adviser, defended the candidate's position.

“Here’s the central point to understand: The White House has been utterly and completely soft on China’s illegal trade practices,” said Navarro, a professor at the University of California, Irvine. "The status quo is the worst of all possible worlds for the United States."

Trump, who was slated to speak in Bangor, Maine, later on Wednesday, took criticism for his trade speech from both sides of the political aisle.

In a call organized by Democrat Hillary Clinton's campaign, U.S. Senator Mark Warner of Virginia, a former businessman and tech entrepreneur, said that while the country needed to do a better job protecting workers, more resources should be put into training them for a new economy.

The Democratic lawmaker criticized Trump's remarks supporting the British decision to leave the European Union.

“The truth is if you are entrusted with positions of responsibility, words matter, your tone matters, your confidence matters and on all of those indicators Donald Trump has failed the test of tone or tenor for leadership,” Warner said.

Clinton held no public campaign events on Wednesday but did announce she would campaign next week with President Barack Obama for the first time this year.

The presumptive Democratic nominee and Trump are almost certain to face off in Nov. 8's general election.

(Reporting by Ginger Gibson and Emily Stephenson; Editing by Jonathan Oatis)