A widely quoted "Department of Agriculture study" that USDA says doesn't exist and an unusual special-interest lobbying campaign by a prominent conservative political group are being used in last-ditch efforts to try to prop open a $1.3 billion income tax loophole for professional commodity speculators.

Congressional and industry opponents of the tax reform measure claim the so-called "USDA study" supports their contention that eliminating commodity trading tax gimmicks will harm the nation's grain marketing system.

"There is no study," says Dawson Ahalt, deputy assistant secretary for economics. The Agriculture Department "raised some red flags" in a private letter to the Treasury Department, he said, but "we've made no study" of the issue.

The threat to the grain markets also has been cited by another opponent of the bill, the National Conservative Political Action Committee, which has been lobbying against the commodity tax measure for more than a month.

Moving for the first time into an issue far removed from NCPAC's traditional conservative causes, Chairman John T. (Terry) Dolan has written three letters to various members of Congress opposing legislation to end the use of certain commodity trading tactics to avoid taxes.

Neither NCPAC nor Dolan has registered with the secretary of the Senate as a lobbyist, a representative of that office said.

A 1946 federal law requires that persons or groups that are paid by others to try to infulence passage for defeat of legislation must register with congressional officials.

The law is loosely worded, and there have been few prosecutions for failure to register, the Senate aide said.

Dolan left Washington for the July Fourth weekend without responding to inquiries from The Washington Post about his role in the commodity-tax debate.

Dolan told The Boston Globe he wrote the letters at the urging of a NCPAC contributor, Peter M. Todebush of Barrington, Ill., a suburb of Chicago. The Globe reported Todebush, a member of NCPAC's executive committee, confirmed the request but did not report when it was made.

Records of the Federal Election Commission show Todebush gave NCPAC a $1,000 contribution in March. FEC regulations require listing the occupation of contributors, but NCPAC failed to provide that information about Todebush.

Todebush told The Globe he is a member of the Chicago Board of Trade, one of two big Chicago commodity exchanges that are leading the opposition to the tax bill.

Dolan's lobbying has introduced politics into whhat had been a bipartisan effort to close a loophole that the Treasury Department says costs the government $1.3 billion a year in taxes.

Congressional Democrats initiated the tax reform measure, but the Reagan administration has endorsed the idea, and a few days ago the Republican-controlled Senate Finance Committee voted 16 to 2 in favor of it.

Dolan has written at least three groups of letters to members of the Finance Committee and the House Ways and Means Committee, which are handling the commodity tax bill.

The letters advocate changes in highly technical provisions of the commodity tax bill, but offer no explanation of how those provisions are related to the conservative ideology on which Ncpac has raised funds from thousands of contributors.

Describing the tax measure as "incredible", "ridiculous" and "truly dangerous," the NCPAC chairman repeated almost verbatim arguments made by the commodity industry's own lobbyists.

The tax reform proposal would "irreparably damage" the nation's grain markets, Dolan claimed and "could conceivably bankrupt" some professional speculators.

Backers of the bill dispute that assessment of the impact on the grain markets but admit it could have a devastating impact on some speculators who for the first time would have been forced to pay federal taxes on all their income.

Using complex techniques called "butterfly straddles, spreads and cash and carry," the commodity traders have found several ways to reduce their tax bills and in some cases to avoid paying taxes entirely.

One trading strategy can be used to convert ordinary income such as salary and investment earnings that are taxed at rates of up to 70 percent into long-term capital gains on which the maximum tax is only 28 percent.

Another technique makes it possible to shift income from one year to the next. By repeating the process year after year, some traders have been able to defer taxes indefinitely.

Under the commodity tax bill, the traders would not only have to pay taxes on their current income, but also could face huge bills for past income that has been deferred.

The commodity tax tricks have been known to industry insiders for at least 30 years, but gained notoriety only recently. The Washington Post reported earlier this year that the technique was promoted by the nation's biggest brokerage, Merrill Lynch, Pierce, Fenner and Smith when Treasury Secretary Donald T. Regan was chairman of that firm.

At his confirmation hearing, Regan promised not to interfere in Treasury efforts to close the loophole. Since then, the Republican Treasury Department has pursued the issue even more aggressively than did the Democrats.

Treasury officials have recommended changes in the bill that go farther than the original measures introduced in the House by Rep. William Brodhaed (D-Mich.) and Benjamin Rosenthal (D-N.Y.) and in the Senate by Daniel P. Moynihan (D-N.Y.).

The Treausury also has urged that commodity trading profits be taxed at a higher rate than provided by the Senate bill. The bill outlaws several sophisticated tax-avoidance schemes, but sets a maximum effective tax rate of 32 percent on all commodity trades. Treasury has called for a 40 percent tax rate.

Commodity industry leaders, who at first claimed tax-motivated trading was an insignificant part of their business, have been forced to change their line because of congressional studies showing the practice is widespread.

About a month ago the industry adopted a strategy of supporting an end to the use of the tax tricks by private investors, but exempting professional commodity traders from the reform provisions.

The industry contends that if professional speculators are forced to pay taxes like everybody else, they will abandon the commodity markets and seek some less risky way to earn a living. Without speculators willing to take the risk of predicting what will happen to grain prices, the whole pricing system on which American farmers depend will be threatened, they contend.

That argument has been advocated by two leading opponents of the legislation, Sen. Steve Symms (R-Idaho) and Marty Russo (D-Ill.), who claim a "USDA study" supports their position.

Symms cited the so-called study during the recent Finance Committee debate, and Russo referred to it in a letter to House colleagues explaining his industry-backed bill.

"The Department of Agriculture recently completed a study" showing that taxing commodity traders' earnings would harm the efficient operations of the grain market, Russo wrote. Along with his letter, Russo provided what he called "preliminary findings" of the "study" to back up his position.

But USDA official Ahalt said the Agriculture Department has done no research that could be considered a "study" of the impact of the tax measure on the grain markets. USDA officials have voiced "our gut feelings" in a letter to the Treasury, he added.

Agriculture officials reportedly spent only two days preparing the letter, which was written by Ahalt's boss, William G. Lesher, assistant secretary for economics, to John Chapoton, Treasury's assistant secretary for tax policy.

Ahalt said the Agriculture Department will not make public the letter because it is an internal communciation between two branches of the Reagan administration. Treasury officials said the administration's position still supports the measure.

Sources who have read both the USDA letter and Russo's version of it say the Illinois congressmen not only overstated the nature of the agriculture department document but also left out important qualifications stressed by agriculture officials.

Russo's letter said the Department of Agriculture "concluded that competitiveness and efficient prices (for farm products) depend on the volume of speculative transactions and, therefore, changes in the tax law for floor traders would increase the short-term volatility of prices and make the outcome of hedging more uncertain and costly."

What the Agriculture Department actually said about taxing floor traders was, "it is possible [emphasis added] that any major reduction in their incentive to trade . . . would increase the short-term volatility of prices and make the outcome of hedging more uncertain and more costly.

"There are no conclusive studies to support this concern," the Agriculture Department letter said, but Russo did not include that statement in his summary.