Twenty-nine senators, backed by a coalition of consumer, environmental, farm and industry groups, yesterday attacked a proposal by Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.) to cut U.S. dependence on imported oil.

The senators and their supporters said the Bentsen plan, contained in the trade bill due to come before the Senate next week, amounts to a back-door approval of an oil import tax that would cost U.S. consumers and industries million of dollars.

Sen. Robert Packwood (R-Ore.) called the measure "one of the most extraordinary grants of power to a president that I have ever seen." He said Bentsen's proposal would give away legislative power to tax, overrule environmental rules or impose rationing.

The differences over the oil provision are likely to provide one of the bitterest battles in the trade debate, pitting Bentsen against a large number of members of his committee. In addition, the Reagan administration has weighed in against the proposal; U.S. Trade Representative Clayton Yeutter characterized it as "sheer protectionism."

"We believe this provision is a well-intentioned but misguided attempt to protect the domestic oil industry," said Sen. Bill Bradley (D-N.J.) and Packwood in a "Dear Colleague" letter. The letter had 27 other signatures yesterday.

Sen. Alfonse M. D'Amato (R-N.Y.) called the Bentsen proposal "special-interest legislation at its worst.

"Any of the bills I have worked for in the parochial interest of New York pale in comparison with this. My efforts were nothing," added D'Amato, who in October held up passage of a $576 billion spending bill and caused the furlough of 500,000 federal workers with a filibuster aimed at giving a New York factory a second chance to persuade the Air Force to buy a jet trainer it didn't want.

Bentsen defended his plan as a way to ease the United States' "growing dependence on oil from the Persian Gulf, which is threatening to drag the United States into the war between Iran and Iraq." He called the argument that his measures extends presidential authority "a red herring."

Bentsen, whose state of Texas has been thrown into economic turmoil by the global oil glut and the fall in prices, pushed his proposal through the Finance Committee by a 12-to-8 vote on the last day of marking up a trade bill. The measure directs the president to submit plans to Congress for reducing the U.S. dependence on foreign oil to take effect when projected imports go above 50 percent of domestic consumption.

The measure would empower the president to pick from a menu of plans to reduce imports, including an oil import tax, import quotas, suspension of environmental regulations, conservation measures, rationing or incentives to domestic producers. Congress could block the proposal with a two-thirds vote of both House and Senate.

Opposition to the measure brought together an unlikely coalition of lawmakers and interest groups, including Sen. Howard M. Metzenbaum (D-Ohio), who said this is the first time in 12 years in the Senate that "I have stood shoulder to shoulder with the U.S. Chamber of Commerce."

Sen. Steven D. Symms (R-Idaho), who said he has been "looking for an issue where I could stand should to shoulder with Howard Metzenbaum," argued that the Bentsen proposal would hurt U.S. farmers just as they are becoming competitive again in world markets. U.S. manufacturing industries -- including steel makers and chemical and plastic companies -- said the measure would increase their fuel costs and make them less competitive.

"Any oil import fee would adversely affect those regions of the country that are particularly dependent on imported oil," said Sen. Gordon J. Humphrey (R-N.H.), who noted that New England gets most of its oil from overseas.