They are an odd couple, David Gartner and Dwayne Andreas.

Gartner - the most trusted aide to one of the most trusted men in the United States Senate. Hubert Horatio Humphrey.For 17 years, he tackled problems pragmatic and political for Humphrey. His reputation on Capitol Hill earned him President Carter's nomination to a $50,000-a year job on the Commodity Futures Trading Commission and the Senate approved him unanimously.

Andreas - one of the richest men in the Midwest, chairman of Archer-Daniels-Midland Co., a $2 billion business that buys corn, soybeans and wheat from country elevators all over the Grain Belt and sells food products all over the world. He remained a low-profile businesman until a $25,000 contribution he made secretly to Richard Nixon wound up in the hands of the Watergate burglars.

What Andreas and Gartner had in common was Hubert Humphrey. For years, Andreas was as loyal to Humphrey with his money as Gartner was with his energy.

Although he hedged one of his political bets by secretly contributing to Nixon, Andreas gave hundreds of thousands of dollars to Humphrey campaigns over the years, traveled with his favorite senator, and sent his daughter to work on Humphrey's staff, where she became Gartner's secretary.

When Humphrey went home to Minnesota for his final campaign against cancer, it was to a home that Andreas helped him obtain. And when Humphrey died, it was Gartner who stood on the front steps to tell the world.

The strength of the bond between the two Humphrey loyalists was revealed when Gartner was nominated May 17 as one of the five members of the Commodity Futures Trading Commission. CFTC regulates the nation's grain futures markets, in which Andreas' Archer-Daniels-Midland buys and sells billions of dollars worth of contracts a year.

In a confirmation hearing before the Senate Agriculture Committee, Gartner brought up his friendship with the ADM chairman, volunteering that Andreas and his daughter Sandra had given ADM stock worth $72,000, in 1975, 1976 and 1977 in irrvocable trust funds to Gartner's four children for their education.

Gartner promised to disqualify himself from any CFTC deliberations having a "direct impact" on ADM, but the stock still troubled some senators. When Henry Bellmon (D-Okla.) said he would not approve the appointment unless Gartner said the stock, Gartner quickly agreed to do so.

Satisfied, the Senate Ag committee sent the nomination to the floor, where it was approved that day, by "unanimous consent" even though only a handful of senators were in the chamber.

Nothing much has changed since then. There have been no new revelations about Gartner or Andreas, no bombshells, no "smoking gun" pointing toward influence-peddling or buying.

But Gartner will go back before the Senate Agriculture committee next Wednesday in what could be his last chance to save his job. Lawmakers and newspaper editorials are calling for his resignation, and President Carter has been forced to defend Gartner's appointment on national television.

What has happened is that the midwestern morality under which Gartner and Andreas thrived has risen up against them. Although East Coast congressmen and columnist have paid little attention to the Gartner affair, the politicians and publishers of the Farm Belt have followed it closely.

The idea of the fox's friend guarding the grain bin has discomfited residents of grain-producing states whose economies rise and full with the price of corn, wheat and soybeans on the Chicago Board of Trade.

As Rep. Berkley Beddell (D-Iowa) puts it."I do not see how anyone can accept $72,000 in gifts from one of the major firms being regulated by the commission on which he serves without the appearance or existence of some conflict of interest."

Congressmen calling for Gartner to step down talk mostly of possible or apparent conflicts but "even the appearance of a conflict of interest damages the CFTC" said Rep. Ed Jones (D-Tenn.), chairman of the House Agriculture Subcommittee on Conservation and Credit.

The subcommittee, which is responsible for overseeing CFTC and its budget, has produced the most damaging criticism of Gartner. A formal letter to President Carter calling for Gartner's resignation was signed by Jones. Bedell, Rep. Floyd Fithian (D-Ind.) and Rep Fred Richmond (D-N.Y.). "Although Mr. Gartner has pledged to disassociate himself from any CFTC decisions which could directly affect his friend's business, we are concerned that the existence of this fund could call into question in the minds of many people the credibility of the commission," they complained to Carter.

The president has yet to respond to the House critics, who wrote him nearly two weeks ago. In a televised news conference the day after he received the critical letter. Carter admitted he did not know personally about the $72,000 gift from Andreas when he nominated Gartner for the CFTC post. But he said he saw no conflict because of "the circumstances (of the gift) and the fact that it had been made known thoroughly."

Despite the president's reassurance, the criticism of the appointment has spread to the Senate and become bipartisan, with Sen. Howard Baker (R-Tenn.), Orrin Hatch (R-Utah), Howard Metzenbaum (D-Ohio), and Jesse Helms (R-N.C.) joining the chorus.

It was Helms who called Gartner back before the Senate Agriculture Committee, where the new CFTC member is expected to be grilled for more thoroughly than he was before.

One aide who will be whispering questions into the ears of Gartner's inquisitors said the going will be tougher because some senators are embarrased about the ease with which the appointment originally was approved. Agriculture Committee members have heard complaints from other senators that the committee should have headed off the controversial appointment before it got to the floor.

Fithian and others have suggested that Gartner's 17 years as top aide to one of the Senate's most esteemed member led senators to skim over the conflict-of-interest question.

Farm state congressmen also complain that urban lawmakers didn't pay much attention to the appointment because they failed to understand the importance of the CFTC and didn't recognize the names of either Archer-Daniels Midland or its chairman.

The CFTC regulates trading in futures contracts in grains and other commodities worth $1.1 trillion a year. A futures contract is the right to buy a commodity at a future date at a fixed price. Food processing companies use them to assure the price and supply of raw materials. Speculators, to try to outguess the fluctuations of the markets.

ADM is one of the biggest U.S. grain dealers and processors, it's own shares are traded on the New York Stock Exchange. As the company said in its latest 10-K report filed with the Securities and Exchange Commission, "We are in only one business - agribusiness."

Rather than selling directly to consumers, ADM sells flour to bakers, corn oil and soybean oil to makers of margarine, and cooking oil and pure grain alcohol to makers of gin and vodka. It also exports grains and food products.

If ADM is little known, its chairman is even less recognized. Andreas' antipathy for publicity - and the press - is legendary in Minneapolis. His profile has become even lower since the story of his secret contribution to the 1972 Nixon Campaign came out during the early days of Watergate.

For reasons has never explained, the long-time Huphrey backer gave $25,000 in cash to Kenneth Dahlberg. Nixon's Minnesota chairman and a national fund raiser for the Committee to Re-elect the President.

In what has become the classic example of laundering campaign contributions, the cash was turned over to Dahlberg in Florida. He flew to Washington with the money and gave it to Hugh Sloan, then treasurer of CREEP. On April 9, 1972, Sloan turned the money over to G. Gordon Liddy. On May 25, Andreas' money, now in the form of a cashier's check, was deposited in the Miami bank account of Bernard Barker, who later was convicted as one of the Watergate burglars.

Andreas also got into trouble for his contributions to Humphrey's campaign. He was indicted - but later found innocent - for allegedly illegally using corporate funds of Interoceanic Corp., an ADM subsidiary, to make a $100,000 contribution to Humphrey.

The wealth and generousity of the ADM chairman is indicated by reports to the Securities and Exchange Commission which show he is paid $325,000 a year and owns more than 1.5 million shares of the company's stock, worth about $22.5 million at today's prices. Hundreds of thousands of other shares, worth several million more, are held in trusts or owned by Andreas' relatives.

The ADM chairman regularly shares his wealth by giving away stock, the reports on his holdings indicate. Although the beneficiaries of the gifts are not disclosed, the size of the gifts indicates that the ones of the Gartner children were among the smaller ones. Whether anyone else in government besides Gartner has benefited from Andreas' largess is a question that troubles critics of the appointment.

Disturbing to them too are the real and potential regulatory questions facing ADM and its broadly diversified subsidiaries.

ADM owns 87 percent of the stock of National City Bank of Minneapolis, and Andreas heads a group of investors who own Ridgedale National Bank in suburban Minnetonka. Nixon fund raiser Dahlberg also is part owner of Ridgedale National, which was chartered in 1972, the year Andreas donated to the Nixon campaign. Despite a rival bid, that charter was granted in only 90 days, near-record time for the Comptroller of the Currency who usually takes four to six months to act.

National City Bank holds the ADM stock that Andreas gave Gartner's children in what Gartner once described as "an irrevocable trust." But when Senator Bellmon demanded that Gartner dispose of the stock, the trustee did so. How could the trust have been called "irrevocable"? the senators will ask Gartner next week.

CFTC officials said that agency has no pending complaints against ADM and has taken no disciplinary action against the firm recently.

As ADM recently revealed to its shareholders, a subsidiary, ADM Milling Co., has been indicted on criminal charges and sued by the government on civil grounds for alleged abuse of the Food for Peace program, under Public Law 480. The indictment accuses the company of "conspiracy to allocate contracts and submit noncompetitive bids on soyfortified sorghum grits" The company faces a maximum fine of $1 million plus civil penalties if it is found guilty.

Also pending against ADM is a 1971 Justice Department case accusing the company of antitrust violations in connection with the purchase of soybean processing plants. Another ADM subsidiary, Supreme Sugar Co., is a defendent in a private class action against the sugar processing industry.

ADM also was involved in recent grain-exporting scandals, with one of its terminal managers and 21 other persons accused of stealing grain from foreign shipments.