Shares in takeover target GrainCorp slumped on Friday following speculation Prime Minister Tony Abbott is reluctant to support the $3.4 billion bid for the grains handler by US grain trader Archer Daniels Midland.
GrainCorp shares fell 46¢ to close at $11.70, after hitting a low of $11.41.
Last week, Treasurer Joe Hockey said he would not be bullied in his deliberations on the bid. Mr Hockey said he would decide by December 17 whether it could proceed.
As Treasurer, he has the right to make the ultimate decision on whether the sale is in the national interest.
Earlier, Nationals leader Warren Truss highlighted the importance of food security, along with grains infrastructure, in the context of the bid. The Nationals are opposed to control of the company going offshore, as are some Liberal MPs in rural seats.
"If we don't own any of the supply chain, it'll be very difficult for us to ever make decisions which can in fact influence whether or not our grain industry is to prosper," Mr Truss said in a recent interview.
Lobby groups such as farmers' federations have also been pushing for the sale to be blocked or, if it is approved, for restrictions to be placed on some GrainCorp assets.
GrainCorp managing director Alison Watkins denied food security was an issue.
"Australia, of any country in the world, has the least concerns of food security," she said on Thursday, discussing GrainCorp's earnings.
The company is the dominant domestic grains handler, responsible for more than half the crop across eastern Australia.
It operates a network of grain ports, most of which are regulated by the Australian Competition and Consumer Commission.
"The Prime Minister must guarantee he will not interfere in this process," shadow treasurer Chris Bowen said.
GrainCorp and ADM executives are expected to appear before a resumed Senate inquiry.
On Thursday, GrainCorp reported a severe profit downturn, posting a net profit of $140.9 million for the year to September, down from $204.9 million a year earlier.
Earnings were hit by the drought in Queensland and northern NSW, with continued dry conditions prompting caution about the near term earnings outlook.
JP Morgan analyst Stuart Jackson told clients on Friday: "The risk of not receiving regulatory approval is low (but not insignificant)."
The prospect of lower grain volumes in the 2014 financial year prompted him to cut his net profit forecast for the company to $111 million from $127.7 million.