After months of relatively diplomatic silence, the Democratic Party's foreign policy hardliners have cautiously joined the ranks of President Reagan critics, firing a few carefully aimed first strikes at the way the new adminstration is going about the shaping of national security policy.

Sens. Henry Jackson (D-Wash.) and Daniel Patrick Moynihan (D-N.Y.) and Dr. Zhigniew Brzezinski, national security adviser to President Carter, used a forum Tuesday sponsored by the Coaliton for a Democratic Majority to criticize the Reagan administration. But their salvos were carefully targeted to cause no damage to the basic hardline national security decisions that the new administration has already undertaken -- and which they themselves support.

"My thesis today is that the new administration has not yet got its foreign plicy act together," said Jackson, the elder patron of the CDM, which seeks to mold its majority coalition around the party's most hawkish wing. ". . . where is the administration going; from here? . . . it strikes me that many fundamental questions have not yet been addressed or resolved."

Moynihan sounded a similar theme. "This is not a foreign policy," he said.

"This is a series of speeches and trips and press statements."

And Brzezinski, who provided the panel with its only detailed tour d'horizon, offered this analysis:

"I think there's [Reagan national security] team, but it has no process and it has no leader." Since World War II, Brzezinski said, foreign policy has been conducted either through a "presidential" format, with the president playing a strong daily role, or a format that has the secretary of state playing a central role.

"I expected President Reagan to fall into the latter category," Brzezinski went on. But the Reagan administration, he maintained, has turned out to be none of the above.

"The president . . . is not deeply engaged in foreign policy. And for a variety of reasons . . . the secretary of state has not been permitted to play that role."

Jackson contented himself mostly with raising questions that he said the administration has failed to answer, rather than providing suggested answers of his own. "Where is the administration's general strategy to deal with the range of threats confronting the Middle East?" he asked.

Turning to Europe, Jackson said: "In one ally after another, the leftwing tail is wagging the government dog, and NATO's deterrent is at a risk. What is the administration's strategy to handle this threat?"

On China, he said: "Will the administration finally get itself together on handling the People's Republic of China? How long will the White House keep the Chinese on knife-edge regarding our policy to sell arms to Taiwan?"

And on the controversial matter of human rights, Jackson asked: "Has the administration, in its eagerness to revise human rights policy, actually endangered some historic sources of strength for the struggle?"

Moynihan was the pointed in his criticism. "I was appalled at the way we have handled ourselves in Asia and Pakistan in the last few weeks," he said. He criticized Secretary of State Alexander M. Haig Jr. for having offered military arms sales to the Chinese and "gotten nothing in return," and he spoke out against the administration's decision to sell F16 jet fighters to Pakistan. Moynihan, former ambassador to India, maintained later in an interview that the Pakistanis should not be considered U.S. allies.

Brzezinski said after his appearance that there are central elements of the Reagan policy that he and other Democratic Party hardliners support enthusiastically -- "especially the tougher tone, and the bigger defense budget."

But he added that it was the appropriate time and place for the Democratic hardliners to offer some criticisms of the Reagan administration.

"A decent interval has passed," he said. "And this was an appropriate forum -- a Democratic Party forum that is more tough-nosed. If this had been a do-gooders' meeting, I might not have spoken out as I did."