Sen. Robert J. Dole (R-Kan.) provided key support to tax legislation benefiting 333 commodities dealers during the last election cycle, when the dealers and their political organizations contributed $70,500 to his political action committee, according to a study by Common Cause.

Dole's support of the 1984 tax bill amendment affecting the dealers represented a turnaround from 1981 when he successfully fought the industry. Now Senate majority leader, Dole was Finance Committee chairman from 1981 through 1984.

Attempts to have Dole comment on the study were unsuccessful.

While most taxpayers must be prepared to prove the legitimacy of a tax deduction when challenged by the Internal Revenue Service, the amendment places the burden of proof on the IRS in dealing with the commodities traders.

In effect, the legislation is designed to create a highly favorable legal framework exclusively for 333 such traders in their dispute with the IRS over the legitimacy of deductions involving past transactions. Common Cause, a private watchdog group, said $300 million or more is involved in the cases, an average of $866,000 per dealer.

House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) also provided essential backing to the controversial amendment. He represents Illinois' 9th Congressional District, which includes part of Chicago, a major commodity-trading center.

In 1981, Dole led the Senate fight to enact legislation restricting commodity straddles," a complex method of taking losses in one year and profits in the next to reduce or eliminate tax liabilities.

During that debate, Dole told his colleagues that commodities traders are "nice fellows . . . and they are great contributors. They haven't missed a fund-raiser. If you do not pay any taxes, you can afford to go to all the fund-raisers."

In the 1981-82 election cycle when the 1981 tax bill was passed, Common Cause found, commodity dealers and their political action committees gave $11,000 to Dole's PAC, Campaign America.

During the next election cycle, however, Common Cause found, the dealers and their PACs gave Campaign America $70,500. During this period, Dole supported the amendment favorable to the dealers.

"What caused Dole to change his tune about the commodity traders between 1981 and 1984?" the study asked. "Dole would not return recent phone calls about his role in the 1984 tax provision, but he has obviously had a firsthand opportunity to see what 'nice fellows' can do for you, if given a chance."

Campaign America is a fund-raising committee established by Dole in 1978. He has used it to transfer campaign contributions of $2,000 to $10,000 to Republican Senate colleagues. In 1983-84, while seeking support to become majority leader, Dole gave $216,150 to Republican senators.

The findings were part of a larger study of the traders' political contributions by Florence Graves and Lee Norrgard, and was published in Common Cause magazine.

The study also found while Dole received hefty support from the commodities interests, most donations by commodity PACs have gone to Democrats.

Among the three largest commodity PACs, the study found that from 1979 to 1984, $237,500 went to House Democrats and $44,000 to their GOP counterparts.

The study contended that a major factor in favoring Democrats was the reported decision of Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee, to "put the word out in 1981 that if Democrats favored the traders during consideration of the tax-straddle loophole, they could be expected to be rewarded."

The study found that contributions from the three largest commodity PACs to the Democratic campaign committee increased from $5,900 during the 1978-80 election cycle to $74,450 in 1981-82, then decreased to $67,500 in 1983-84.