At least six and possibly seven Democrats on the 17-member Senate Foreign Relations Committee "have tentatively made up their minds" to vote against Kenneth L. Adelman as director of the Arms Control and Disarmament Agency, and three Republicans also are undecided on the nominee, sources said yesterday.

The nomination, therefore, could be in some danger.

One Democratic member said after the Democrats caucused yesterday morning that "it will take virtually solid Republican support to get him through the committee."

But a committee Republican said three GOP members--Sens. Charles McC. Mathias Jr. (Md.), Rudy Boschwitz (Minn.) and Larry Pressler (S.D.)--remain "uncommitted" on the nomination.

Their worry is that Adelman, chosen to head the disarmament agency after former director Eugene V. Rostow was fired last month, has neither the experience and stature nor the commitment to arms control needed for the job.

Rejection of the nomination would be a blow to President Reagan and an implicit vote of no confidence in his arms control policy.

An administration official said that the White House now recognizes it faces "a major battle" on the nomination and is "reorganizing the support staff" that prepared and accompanied Adelman, 36, to his rocky first confirmation hearing last Thursday. For example, Powell Moore, assistant secretary of state for congressional affairs, will be on hand this Thursday when Adelman returns.

"I think he'll be confirmed," the administration official said, "but the big hurdle is the committee, whose center of gravity is a little left of center."

And after yesterday's Democratic caucus, Sen. Alan Cranston (D-Calif.), who has led the opposition to Adelman, said he thought the nomination might be voted down.

He said he believed all the eight Democrats on Foreign Relations except Sen. Edward Zorinsky (D-Neb.) would vote against Adelman.

Cranston, Senate Democratic whip, said the Democrats want to send the White House two messages: that "Congress is serious about arms control" and that the president should stop putting people in charge of agencies whose duties they oppose, such as "[Secretary James G.] Watt at Interior and [Anne M.] Gorsuch at EPA," the Environmental Protection Agency.

Sen. John Glenn (D-Ohio) was among the Democrats at the caucus prepared to vote against the Adelman nomination, according to an aide. Glenn, who along with Cranston is a potential presidential candidate, said he believes the ACDA director "is a signal to the world about our sincerity toward arms control . . . and should be the advocate for arms control within the administration," his aide said. "He doesn't think Adelman can do that."

The confirmation hearing also may become the forum for a broader discussion of administration arms control policy.

This could occur when Rostow testifies. He originally was scheduled to testify last Thursday, but balked, reportedly because he did not want to appear on the same day as his named successor. He now is expected to be heard this week, and is bound to be asked about administration policy and internal administration splits on arms issues.

Adelman's nomination originally had been expected to encounter little opposition. But that was before the deputy U.S. representative to the United Nations appeared at his first confirmation hearing. His performance "was so bad," one member said yesterday, "that members just can't sign on for it."

According to a Democratic analysis of the transcript, Adelman answered at least "20 times, 'I don't know' or 'I hadn't thought about that' to questions directly related to arms control issues," a committee aide said. He is "bright and capable," one top administration official said yesterday, "but he was a little apprehensive and did not perform particularly well."

Adelman was asked to return for more questioning by Boschwitz, who said last week he had not had time to review the substantial amount of writing the nominee had done, particularly articles critical of the Carter administration's strategic arms limitation treaty (SALT II).