Opposition from the State and Defense departments has thwarted a White House idea for an "arms control czar" to break interagency deadlocks on U.S. negotiating offers to the Soviet Union, and no substitute to expedite policy-making is in sight, administration sources said yesterday.

"It hasn't sold," a White House official said of the "czar" idea, which was endorsed shortly before the election by White House chief of staff James A. Baker III and is said to have been backed in some form by national security affairs adviser Robert C. McFarlane.

The two most powerful Cabinet members, Secretary of State George P. Shultz and Secretary of Defense Caspar W. Weinberger, made plain in recent interviews that they oppose naming a high-powered official to break what many have described as an interagency "gridlock" on arms control issues between their departments.

In different language, Shultz and Weinberger indicated acceptance of the idea of a "special envoy" who might take a leading role in across-the-board arms discussions with the Soviets, but who would speak and act on instructions hammered out through the existing interagency process.

A White House official conceded that an envoy with such limited powers might be "little more than a messenger" and probably could not resolve internal disagreements.

The official said President Reagan does not appear to have sorted out how arms control policy is to be made in his second term, with most or all of the previously contending policy-makers staying in their jobs.

With the policy-formation process still up for grabs, the administration nevertheless welcomed indications of Soviet willingness to hear more about the proposal for "umbrella" arms talks that was presented by Reagan to Soviet Foreign Minister Andrei A. Gromyko in Washington discussions Sept. 28.

White House spokesman Larry Speakes, commenting on statements here Wednesday by a senior Soviet Embassy official, said, "We would like to continue to outline for them the Soviets our proposal for the umbrella approach to these talks."

He added, "We think it would be timely to discuss the relationship between offensive and defense systems and also to look toward the possibility of renewing the long-range and intermediate-range missile talks, as well as continuing talks in a number of other areas."

Speakes did not mention talks on halting tests or deployment of antisatellite weapons and other arms in space, which have been at the top of the Soviet list for negotiation. Weinberger, in a Los Angeles Times interview published yesterday, stressed his opposition to a moratorium on antisatellite tests, saying that "the antisatellite capability can be a very important one, and we know of no credible way of verifying a moratorium."

In interagency discussions, the State Department reportedly has favored a moratorium under certain conditions. A White House official said the administration position remains "fuzzy."

He added that there is agreement on one point: any moratorium would have to grow out of negotiations with the Soviets rather than precede it.

The Soviet diplomat, who asked that his name not be used as he spoke to reporters at an embassy reception Wednesday, said the Kremlin is awaiting specific information about the U.S. proposal for the umbrella talks. The official called the umbrella idea unprecedented in Soviet-American arms negotiations and expressed uncertainty about what it would mean in practice.

Speakes told reporters that "the entire idea has not been fleshed out in talks with the Soviets" but that the administration "would like further opportunity to do so."

A U.S. official in the arms control field said there is no precision about the umbrella plan even in the administration. "We're as confused about it as they the Soviets are. Nobody know what it means," he said.

Another official defined the umbrella approach as "a framework which is interlocking, wherein all arms control subjects are interrelated in some way."

Asked about details, he said the administration was flexible about them while trying to find out what the Soviet Union would find acceptable.