The chairman of the Senate Finance Committee on Monday warned he will not support a proposed free trade agreement between the United States and the European Union unless it tears down barriers that have long blocked U.S. farm exports.

“As chairman of the committee overseeing U.S. trade, I will support a deal only if it gives America’s producers the opportunity to compete in the world’s biggest market,” Senator Max Baucus, a Montana Democrat, wrote in the Financial Times.

With the U.S.-EU trade talks expected to begin by June, Baucus also urged President Barack Obama, a fellow Democrat, to replace outgoing U.S. Trade Representative Ron Kirk with an experienced leader who has “proved mastery of the details of trade negotiations.”

Baucus aides did not reply to a request for more detail, including whether he had intended to discourage Obama from picking White House acting White House budget director Jeffrey Zients, who some see as a top candidate for USTR even though he does not have a trade background.

U.S. farm groups, in a letter on Monday to the U.S. Trade Representative’s office, also underscored their insistence that the proposed U.S.-EU agreement “cover all significant barriers in a single comprehensive agreement.”

The 64 groups said they objected to the idea contained in a U.S.-EU working group report the pact could “evolve over time” by eliminating most barriers to trade and investment and setting up a mechanism to address more contentious issues.

Obama announced plans to negotiate a free trade agreement with the European Union last month in his annual State of the Union speech, following more than a year of preliminary talks between the two sides. Formal negotiations are expected to begin by June.

The proposed “Transatlantic Trade and Investment Pact” would be the biggest trade deal since the World Trade Organization was founded 20 years ago, and Baucus wrote that “it must be treated as a top priority for the U.S. administration and Congress.”

Baucus, like many lawmakers from farm states, long has been frustrated by European restrictions on agricultural practices approved in the United States, such as the planting of genetically modified crops and the use of the feed additive ractopamine to produce leaner beef, pork and turkey meat.

“I heard that there is some interest on the European side in pursuing a limited agreement that would set aside tough issues in order to conclude a quick deal on the easier ones. That is a recipe for failure,” Baucus said.

“Any bilateral trade and investment agreement must be comprehensive and address the full range of barriers to U.S. goods and services if it is to receive broad, bipartisan congressional support,” he said.

Baucus said the Finance Committee would meet in coming days “to discuss how we can work together to accelerate” the U.S.-EU talks.

EU Trade Commissioner Karel De Gucht on Saturday said resolving differences in regulatory approaches to food and other goods would be the most difficult part of the U.S.-EU talks.

“But if we are pragmatic, creative and realistic it will certainly not be impossible,” De Gucht said in a speech at the Harvard University’s John F. Kennedy School Of Government.

(Reporting by Doug Palmer; Editing by Vicki Allen and Jackie Frank)