Deputy Secretary of State Kenneth W. Dam said yesterday that the new MX missile would be "available for negotiations" in U.S.-Soviet nuclear arms talks but it was in no sense a "bargaining chip" built to be traded away and Moscow would have to "give up something equally important" for the MX to be dropped.

Appearing before the Senate Foreign Relations Committee, the State Department's second-ranking official came up against further denunciations of administration arms control policy and annoyance over a letter sent the committee by Kenneth L. Adelman, director of the Arms Control and Disarmament Agency.

It was the third day in a row the panel has been stirred up over the Adelman letter, which suggested that the administration would give up its planned force of 100 MX missiles only if the Soviets dismantled all 650 or so of their latest SS18 and SS19 missiles.

Sen. Charles McC. Mathias Jr. (R-Md.) led off the hearing by telling Dam that both the letter and earlier testimony by Adelman in secret session "lend support to the contention of those who argue that the administration has made impossible demands on the U.S.S.R. in its START Strategic Arms Reduction Talks proposal."

"Demands that the Soviets will not accept or consider seriously," Mathias said, "may jeopardize the arms control process itself. What if by setting standards that can't be met, we have doomed ourselves to continuing the escalation of nuclear weaponry and worse?" he asked.

Mathias said the Adelman statements "bring into serious question the basis on which at least some of us voted" with President Reagan to support MX, thinking it was something of a bargaining chip in negotiations.

Dam sought to defuse the Adelman letter by insisting that it was "a hypothetical answer to a very hypothetical question" about what the United States would consider trading for MX and that no such proposal had been made to the Soviets. Dam declined to say what kind of a deal might be acceptable and said it would not be wise to discuss that or forecast how the arms talks might turn out.

But he did not back too far away from the Adelman letter, suggesting that the MX might be negotiable, but only for a high price.

He said "bargaining chip" is a "very treacherous" word, one the administration has not used concerning the MX, which it maintains is needed to modernize U.S. forces and as "leverage" to induce Moscow to bargain.

The president, however, has also talked about "everything being on the table" at Geneva and it is the apparent conflict between these two positions that has led some lawmakers to feel they were duped by the White House.

Dam stressed that the 10-warhead MX is "a vital aspect of our national security" and that no part of the existing U.S. START proposal at Geneva prohibits the planned 100 missile deployment.

The U.S. proposal, he said, is a "comprehensive package" reducing the total number of all missiles, warheads and "destructive potential" in each arsenal. But if Moscow decided it wanted to, he said, "we are prepared to talk not only about numbers but also about the composition" of specific forces allowed on each side.

"In that sense," he said, "the MX is available for negotiations. But it is not a bargaining chip in the sense that we are just deploying it for the purposes of negotiation."

Dam also said Moscow has thus far "not shown any interest in discussing" specific weapons, but rather overall force levels with each country remaining free to decide which weapons to field within those limits.

The U.S. START proposal does contain demands that would specifically limit the current force of medium and heavy Soviet multiple-warhead missiles to 210. The U.S. plan contains no restrictions on any new U.S. missiles.