The ranking minority member of the Senate trade subcommittee yesterday threatened to hold the new multilateral trade agreement for ransom until President Carter creates a separate Department of Trade.

The statement by Sen. William V. Roth Jr. (R-Del.)-the man most likely to lead GOP Senate support for the trade treaty-came as a special White House task force was finishing an option paper for the president on ways of bolstering U.S. trade policy. But none of the recommended options reportedly includes the creation of a new department.

Upset that his proposal is apparently not being taken seriously at the White House, Roth chose to press his point.

"Whether I can support the (trade) agreements will depend on whether the president supports a new department," the senator said in an interview.

The agreement, initialed last month in Geneva by the United States and 22 other nations, calls for a reduction in tariffs and other trade barriers over the next decade. Failure by the U.S. to ratify it would certainly disrupt the entire agreement and possibly prompt protectionist countermeasures by America's trading partners.

In a related action, the leading business advisers on the trade talks also called yesterday for a thorough reorganization of the government's trade-support structure.

Robert Galvin, chairman of Motorola Inc. and head of the Industry Policy Advisory Committee on trade, announced the committee had voted unanimously for "a strong, centralized high-level executive branch structure" to be set up to enforce the new agreement and promote U.S. trade interests abroad.

A majority of the committee's members favor establishing a new Department of Trade, Galvin said.

Official reaction at the White House was reserved. Senior officials offered assurances of changes in the way U.S. trade policy is devised and conducted-though the changes, they said, are not likely to result in a new department.

"There will eventually be some substantive restructuring of trade responsibilities, not just a shifting of agencies," stated Special Trade Representative Robert S. Strauss, whose office did the tough negotiating for the trade package.

Strauss said he is still confident that Congress will approve the trade agreement without a serious hitch-and that Roth will end up supporting the agreement.

Responsibility for U.S. trade policy is currently divided among a handful of departments and agencies. The Commerce and State Departments promote exports, the Treasury Department and the International Trade Commission protect against unfair import competition, the Export-Import Bank helps finance exports, and Strauss' office is in charge of special trade negotiations.

This splintering of authority, say its critics, results in poor policy coordination at home and weakened impact abroad-just at a time U.S. businesses are being challenged most vigorously in foreign markets by Western European and Asian countries.

Roth, together with Sen. Abraham Ribicoff (D-Conn.), has introduced a bill to consolidate U.S. trade functions in a new Department of Trade-noting in the process that every other industrialized nation has one. Senate Majority Leader Robert Byrd (D-W.Va.) has put forward a trade department bill of his own. And the Senate Finance Committee endorsed the idea last week.

But the agencies which would be affected are less than enthusiastic about the idea of a separate department, and on Capitol Hill and in the business community the proposal has drawn mixed reviews.

Politically, many say, the idea simply isn't achievable. Too many agencies must be asked to give up too much treasured jurisdiction.

Moreover, after creating the controversial Department of Energy-and with two other new departments currently under consideration, one for natural resources and one for education-Congress and the White House are showing themselves reluctant to push for yet another.

"There's not a lot of support for a new department here." said one official in the Office of Management and Budget. However, the official added that there is solid backing for a consolidation of some trade functions into a new agency within the Department of Commerce.

"There are lots of possible units that could be moved," said the official, indicating that Carter could decide as early as this week on which agencies will be affected. "We're at a sensitive point in the decision-making process."

But Roth argued against the Commerce Department compromise. "To do the job, we can't put (a new trade office) in bureaucratic Commerece Department," the senator said. "It's one of the most lethargic agencies we've got."

Roth said that the new trade agreement would not be acceptable without a new department to make sure the agreement is properly enforced and to guard against changes in the trade rules which would be contrary to U.S. interests.

Said Motorola's Galvin: "We need something much better then we've ever had before." CAPTION: Picture, SEN. WILLIAM V. ROTH JR. . . . presses his point