President Carter's controversial decision to support "cargo preference" legislation stirred a partisan furer on Capitol Hill yesterday with GOP charges that it represents a White House "payoff" to the maritime industry.

The chief administration spokesman for the measure. Assistant Secretary of Commerce Robert J. Blackwell, was caught in a crossfire of accusations concerning a $100,000-a-year job offered him in recent weeks by a group of shipping companies.

Blackwell, who heads the Federal Maritime Administration, acknowledged yesterday morning that he was thinking of taking the job, but he insisted that there was no more than "the specter of an impropriety" hanging over it.

Speaking with reporters following a Senate hearing on the administration's "cargo preference" plan, Blackwell maintained that he had successfully insulated himself from decisions affecting his prospective employers since the position was first offered him on June 22. He said Secretary of Commerce Juanita Kreps apparently "acquiesced" in his judgment after being notified of it in a June 24 memo.

Blackwell hotly disputed charges by Rep. Paul N. McCloskey (R-Calif.) that his offer to head a prospective trade association of none ocean freight companies may have been connected with his opposition to a bill affecting those companies and U.S. maritime subsidies. The Maritime Administration chief denounced the suggestion as "the dirtiest, lowest thing that has ever happened to me in 23 years of government service."

The Senate hearing on Carter's proposal to guarantee U.S.-glag oil tankers nearly 10 per cent of the oil import market also ended on an acrimonious note. Repeatedly, Sen. Ernest F. Hollings (D-S.C.), who persided as acting chairman of the Senate Commerce Committee, derided complaints by Sen. Robert P. Griffin (R-Mich.) that the bill was being hurried through in "slipshod fashion."

Mr. Secretary, I'm sorry you have to put up with this nonsense," Hollings said to Blackwell at one point. "What's made this hearing slipshod is his [Griffin's] attesdance here this morning."

Griffin noted that the maritime unions pushing for cargo preference legislation (such as the Marine Engineers Beneficial Association) contributed more than $100,000 to Carter's primary campaign and gave more than $990,000 to candidates for the House and Senate. He also cited a June 24, 1977, memo to the President from Carter's special representative for trade negotiations. Robert S. Strauss, in which Strauss counseled:

"Politically, something in the way of a cargo preference is going to be very hard to resist."

Griffin said he thought Strauss should be called to testify, along with a series of administration officials strongly opposed to "cargo preference" legislation, such as Treasury Secretary W. Michael Blumnethal. Hollings said he had no intention of doing that.

"I'm shocked," Grifftin said.

"Stay shocked," Hollings replied.

"I'm bored to death with this charade. It's time to get this country moving again and build up our merchant marine."

The maritime industry has been pressing for a bigger share of the oil import market - as much as 30 per cent by 1980 - but opponents have charged that even the President's scaled-down plan would cost the American public billions of dollars in higher petroleum prices.

In his testimony, however, Blackwell insisted that the measure would enhance "the national security . . . at a very modest cost" - no more $180 million a year in additional transportation costs.

In a "Dear Colleague" letter, McCloskey said that the nine firms offering the job to Blackwell have "applied for operating subsidies administered by Mr. Blackwell, and all strongly support the proposed cargo preference legislation."

Blackwell told reporters that the possible trade association job was first mentioned to him June 22 during a visit on another matter by Jim Amoss of New Orleans, president of Lykes Brothers Steamship Co. After conferring the next day with Commerce Department general counsel Sam Nemcrow. Blackwell said he decided to notify Secretary Kreps and to disqualify himself as chairman of the three member Maritime Subsidy Board which makes most of the decisions affecting the nine companies.

Several weeks later - apparently on July 13 - just before leaving on a trip to Florida, Blackwell said he met briefly with Jim Horn, president of American Export Lines of New York. He said Horn "indicated to me they were considering a salary range around $100,000 and a contract for three years."

Blackwell said he expects to make a decision within the next week. Asked why he thought he was offered the job, he said: "They think I'm a good man . . . I know this industry."