One of the defining actions of President Donald Trump's tenure in the White House so far is his decision to withdraw the U.S. from a 12-nation trade pact that would have had wide-ranging implications for the global economy.

Now that deal — the Trans-Pacific Partnership — looks like it's going to be settled without the U.S.. That may mean American farmers and small business owners will soon regret Washington's exit.

"The U.S. business community that's going to get hit the hardest when TPP happens is going to be agriculture and small and medium-sized enterprises," said Steve Okun, senior advisor at international trade consultancy McLarty Associates and a former deputy general counsel at the U.S. transportation department.

Progress on the agreement, now known as the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, hit a milestone over the weekend as the 11 remaining members agreed on core elements for a new framework without the U.S.

Once the new deal is implemented, American farmers could be left at a disadvantage: The agreement seeks lower tariffs for goods traded among members, the bulk of which are Asia-Pacific nations. That means non-member countries, such as the U.S., will still face high rates when shipping to Asia-Pacific.

"It's going to really benefit farmers and agricultural producers here in Asia-Pacific and not the U.S.," Okun told CNBC.