 WASHINGTON (Reuters) - With talks to renegotiate the NAFTA trade pact just weeks away, U.S. farm groups and lawmakers from rural states are intensifying lobbying of President Donald Trump’s administration with one central message: leave farming out of it.  Trump blames the North American Free Trade Agreement - the “worst trade deal ever” in his words - for millions of lost manufacturing jobs and promises to tilt it in America’s favor.  But for U.S. farmers the 23-year old pact secures access to stable, lucrative markets in Mexico and Canada that now account for over a quarter of U.S. farm exports.   Now they fear this access could become a bargaining chip in efforts to get a better deal for U.S. manufacturers.  “Perhaps some other sectors of our economy are given better terms and in exchange for that agriculture tariffs would be reintroduced,” said Joe Schuele, a spokesman for the U.S. Meat Export Federation in Denver, Colorado.  Another concern is that the mere uncertainty of open-ended trade talks could drive Mexico to alternative suppliers of grains, dairy products, beef and pork.  Mexico became even more crucial after Trump’s pullout from a vast Pacific Rim trade pact negotiated under Barack Obama dashed farmers’ hopes of free access to more markets.  Next week, U.S. Trade Representative Robert Lighthizer is due to outline the administration’s goals for the NAFTA talks to Congress and the farm lobby has turned up the heat in the past weeks to ensure that its interests will make Lighthizer’s list.  Operating under the umbrella of the U.S. Food and Agriculture Dialogue for Trade, more than 130 commodity groups and agribusiness giants since Trump’s inauguration have been bombarding the new administration with phone calls and letters, public comments to USTR and face-to-face meetings with top officials who have Trump’s ear.  “Our first ask is to do no harm,” said Cassandra Kuball, the head of the umbrella group.  Lobbyists said that Lighthizer, Agriculture Secretary Sonny Perdue and Commerce Secretary Wilbur Ross have been receptive, but the wild card is how Trump ultimately will come down on the talks. They also wonder what concessions Mexico will seek from Washington in the talks due to start in mid-August.  Among the groups involved are the American Soybean Association, Corn Refiners Association and National Grain and Feed Association and firms such as Land O’Lakes, Inc., Tyson Foods(TSN.N), Inc., Louis Dreyfus Company North America, Archer Daniels Midland Co. and others.  For example, U.S. cotton producers, marketers and shippers in mid-June warned the Trump administration that any weakening of NAFTA “would threaten the health of the U.S. industry and the jobs of the 125,000 Americans employed by it.”  Annual U.S. farm exports to Mexico have grown from about $4 billion in 1994, when NAFTA began, to an estimated $18.5 billion this year. With Canada included, that number is forecast to reach $40 billion, quadrupling under NAFTA.  Republican lawmakers from rural states that have backed Trump in the 2016 election have sought to leverage their political clout to press farmers’ case at a time when they struggle with low crop prices.  Pat Roberts, Republican senator from Kansas, who chairs the Senate Agriculture Committee, said he used an unexpected invitation for a private White House meeting with Trump to plug in agriculture’s cause in NAFTA and beyond.  “He (Trump) wanted to know what was happening in farmland,” Roberts said. “I told him we went through a very rough patch and if we did not have a strong, robust, predictable trade policy, it’s going to make life much more difficult in farm country,” Roberts said of the 45-minute meeting in late June.  In May, 18 Republican senators, mainly from pro-Trump farming states, wrote the administration about the “tremendous growth” in U.S. trade with Mexico and Canada as a result of NAFTA.  “Efforts to abandon the agreement or impose unnecessary restrictions on trade with our North American partners will have devastating economic consequences,” they warned.  Trump’s pledges to crack down on immigration and calls for a wall along the border with Mexico also vex farm state lawmakers.  “What I really need is a good, solid immigration system,” South Dakota Republican Senator Mike Rounds said. Given his state’s low unemployment rate of just around 2.8 percent, farmers and ranchers need better access to legal foreign labor, he said.  Agriculture Secretary Perdue got a taste of farmers’ angst when met cattle ranchers in Nebraska on May 20. The event was held shortly after Washington agreed with China to resume beef exports, but some 60 ranchers who gathered at U.S. Senator Deb Fischer’s Sunny Slope Ranch quickly turned to NAFTA.  “If the president wants to renegotiate that agreement with our neighbors and partners in Mexico and Canada please leave the ag portion of that discussion out,” said Pete McClymont, executive vice president of Nebraska Cattlemen, summarizing the discussion.  While lobbying in Washington, some Republican lawmakers have also met with Mexico’s ambassador and U.S. farming representatives traveled south to assure their partners unsettled by Trump’s “America First” mantra.  “The common comment is: ‘why are you here? The problem is not with us. The problem is in Washington. Why are you talking to us?’” said Tom Sleight, president and CEO of the U.S. Grains Council. “The new normal is that feed buyers, millers, grain buyers are actively looking at alternative sources,” he said.  It will take months to find out how effective the lobbying was. Meantime, some are willing to give Trump the benefit of the doubt.  Daryl Haack, a corn and soybean farmer from Primghar in northwest Iowa, like others fears retaliation from either Canada or Mexico, but is optimistic it will not come to that.  “I think President Trump is a negotiator,” he said. “I think he runs bluffs. A lot of negotiators will do that.”  