In their harshet attack on President Carter since he took office, top Republican leaders assailed, his support of cargo-preference legislation as "a blatant political payoff" to the maritime industry and vowed to step up their demands for a congressional investigation.

Republican National Chairman Bill Brock, Senate Minority Leader Howard H. Baker Jr. (R-Tenn.) and House Minority Leader John J. Rhodes (R-Ariz.) protested in a joint statement that the administration has already refused to permit the testimony of key government officials opposed to the cargo-preference proposal.

The three GOP spokesman maintained that a congressional inquiry would be in full flower by now if a Republican President had made a similar decision "in furtherance of a campaign promise."

White House press secretary Jody Powell dismissed the complaints as "partisan rhetoric." He maintained that the President's support of the maritime measure was "in the country's interest."

Powell also vigorously defended Gerald Ratshoon, President carter's campaign advertising director and still an official media adviser against charges of impropreity stemming from a $500,000 advertising campaign that the maritime indsutry commissioned this spring on behalf of cargo preference.

Alluding to a memo to the President from Secretary of the Treasury W. Michael Blumenthal, the GOP leader charged that the plan could cost customers more than $800 million lion a year by 1985.

Ending weeks of uncertainty, the President last month decided to support a bill guaranteeing U.S. flag tankers nearly 10 per cent of the nation's oil import market. The legislation is now before the House Merchant Marine and Senate Commerice committees.

The Republicans said that Carter, who received more than $100,000 in campaign contributions from U.S. maritime interests last year, ignored the recommendations of most of his major advisers and the protests of 11 maritime nations in endorsing the cargo-preference principle. At present, only about 3 per cent of the nation's oil imports, which now total some 8.4 million barrels a day, is arriving here on ships flying the U.S. flag. Transporting it under foreign flags is much less expensive.

Baker called the situation "a budding scandal which must not be permitted to be covered up." He said congressional Republicans will do "everything in our power in both houses" to obtain the testimony of Blumenthal and other administration witnesses who counseled against any form of cargo preference.

The State Department, the Council of Economic Advisers and apparently the Defense Department were also opposed to the proposal.

"Either those witnesses are going to come and testify or we're going to make them wish they had," Baker declared yesterday morning at a press conference on the issue.

House Merchant maring Chairman John M. Murphy (D-X.Y.) and acting Senate Commerce Committee Chairman Ernest F. Hollings (D-S.C.) last week turned aside Republican efforts to summon various administration witnesses, including Robert S. Strauss, the President's special representative for trade. In a June 24 memo to the President, Strauss had advised: "Politically, something in the way of a Cargo Presference is going to be very hard to resist."

GOP National Chairman Brock accused Hollings in particualr of an "arrogance and autocratic approach that is just unbelievable." Brock also critized the fact that the administration's only witness on the issue, Assistant Secretary of Commerice for Maritime Affairs Robert Blackwell, was recently offered a $100,000-a-year job by a group of nine ocean freight companies.

"That just smells," Brock charged. "The administration has said there is no impropriety. I can't imagine that."

Blackwell has said he immediately reported the tentative job offer, which he is still considering, to Commerce Secretary Juanita Kreps and disqualified himself from further decisions of the three-member Maritime Subsidy Board. He said the board is the focal point for "almost" all maritime decisions affecting the nine companies. They carry dry cargo and would not directly benefit from oil cargo-preference legislation.

Powell said Kreps assured the President at yesterday's Cabinet meeting that Blackwell had "insulated himself from conflicts of interests in the matter."

Asked about other witnesses the GOP wants to hear, Powell said he knew of no "requirement, once we take a position, of furnishing witnesses" for an opposing point of view.

Adman Rafshoon's agency was hired by the U.S. Committee to Turn the Tide - a coalition of shipbuilders and allied interests - to drum uo nation-wide support for cargo-preference legislation. Rafshoon, in turn, hired Carter pollster Pat Caddell's firm to perform some market testing on the ad campaign's impact.

Powell said flatly, however, that "Gerald Rafshoon was not involved in the President's decision on this matter." Although Rafshoon has White House credentials and easy entree. Powell added that "I don't know of any policy decision he's been involved in."

The Republicans said they were determined not to let the matter drop. "This does not signal a declaration of war against Jimmy Carter," Sen. Baker said, "but it does say the Republican Party, as the loyal opposition, is doing what it is supposed to do."