Republican presidential candidate Donald Trump has fired back at the US Chamber of Commerce, saying the nation's largest business association needed to "fight harder" for American workers.
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 this week with Mr Trump's vocal opposition to international trade deals, calling his proposals "dangerous" ideas that would push the United States into another recession.
Mr Trump struck back the following day, saying the organisation 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 tweeted.
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 (TPP) among 12 Pacific Rim countries, including Australia.
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 Mr Trump still struggles with uniting his party.
But fighting against trade deals has proven successful for Trump among voters concerned about the loss of manufacturing jobs.
Peter Navarro, a Mr 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.
In a call organised by Democrat Hillary Clinton's campaign, US 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 criticised Mr Trump's remarks supporting the British decision to leave the European Union.
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.