Customs Commissioner William von Raab's directive that his agents begin seizing passports of suspected drug smugglers later this month was greeted with mixed skepticism and surprise by government officials and civil liberties lawyers yesterday.

They said the plan seems likely to face the same uphill fight that killed two other controversial von Raab drug-fighting proposals, government officials said yesterday.

Von Raab's apparently unilateral decision to have Customs Service officers begin confiscating passports of suspected drug smugglers on March 15 encountered a buzz saw of skepticism.

"It's pretty far out. . . cowboyish," said Rep. Don Edwards (D-Calif.), who demanded von Raab explain the legal basis for his directive. Phyllis E. Oakley, a State Department spokesman, also expressed concern over "serious constitutional issues" the proposal raises.

Separately a report issued yesterday by the RAND Corp. gave dim prospects to hopes government agents can cut the flow of cocaine and marijuana into the United States.

The report, prepared for the Defense Department, cautioned that increased efforts to interdict cocaine smuggling are unlikely to affect the drug's wide availability and it warned that using more military forces against drug traffickers probably won't cut into illicit drug usage. That finding is likely to bolster contentions of Defense officials who say the armed services should not become heavily engaged in the fight against drug smuggling.

"While interdiction raises smugglers' risks, they adapt in the short run and learn in the long run," said Peter Reuter, the report's author. He wrote that large increases in funds for patrol forces might boost the number of seizures and arrests, but they will not curtail consumption.

"The rather pessimistic tone of this report should not be interpreted to mean that interdiction is useless or that military services should not participate," the report said. Interdiction serves as an equitable deterrent to the smugglers and it signals to other countries that the United States is determined to slow the flow of drugs, it said.

On the von Raab passport proposal, a State Department official simply responded with prolonged laughter when told the Customs chief had advised his agents to tell complainers to address their concerns to the department's K Street passport office.

"Tell me, honestly, does this have a prayer?" said a lobbyist for the American Civil Liberties Union (ACLU). Susan Brenda, the lobbyist, said the proposal is certain to be challenged in court.

But von Raab, a staunch Reagan administration supporter, said through a spokesman yesterday he intends to proceed with the policy even though some may doubt its legal basis. Ed Kittredge, von Raab's spokesman, said the commissioner had directed Customs officers to begin seizing the passports of individuals "who smuggle drugs of any quantity into the United States."

Kittredge decribed the proposal "as another logical step" after rejection of the commissioner's earlier proposal to mark the passports of suspected drug smugglers, but declined to make the text of von Raab's confiscation order public.

The spokeman acknowledged, however, that the order did not include a statutory basis for taking the passports, which Kittredge said would be "seized as evidence of a crime." Seized passports will not be forwarded to the prosecutors in any accompanying drug case, but will be sent to the State Department's passport office in Washington, he said.

State Department officials recently rejected as unconstitutional von Raab's proposals to mark passports and White House officials said they never gave serious consideration to the commissioner's suggestion that Customs agents be allowed to shoot down aircraft suspected of flying drugs into the country.

In announcing his new passport seizure policy before a White House conference on drugs, von Raab acknowledged that it was opposed by the State Department. Even so, he declared to applause, he was ordering it implemented.

Von Raab decried his opponents as "what I call conscientious objectors. They don't want to get with the program. And there are people who can find a legal reason for not doing just that or anything. . . . In this case there are some low-level types at the State Department who don't like the idea of us taking the passports or stamping them to indicate that someone is shipping drugs into the United States."

State Department officials were surprised by the announcement, which one described as "a new wrinkle on the old package" von Raab had previously offered. But that same official said the department was not rejecting the idea. "We're taking it very seriously," the official said.

"We've all got to look at new ideas," said Oakley. She said she believed the department's lawyers could research the issue in the 12 days before the policy is to be implemented. Under federal law , the secretary of state is the only official who can revoke a U.S. passport.

Rep. Edwards said the House constitutional rights subcommittee he heads would investigate the order. He suggested it violates "due process" guarantees for a hearing into passport confiscations, a point echoed by ACLU lobbyist Brenda.

Brenda said the Supreme Court held in a 1981 case that the government could revoke a passport of individuals who "are causing or are likely to cause serious damage to the national security or foreign policy of the United States." Neither is likely in the case of an individual suspected of drug smuggling, she said.