The relevant congressional committees have served notice that they intend to blow the lid off President Carter's new defense budget by adding some $2 billion to it, and the House Appropriations chairman doubts he can stop it.

The chairman, George H. Mahon (D-Tex.), said in an interview yesterday that he cannot recall a previous drive with as much steam behind it.

"I am more or less committed to holding the line" at the president's requested total of $126 billion said Mahon, but added he "would anticipate" additions despite his own efforts to hold the line.

"People are just a little bit uncomfortable," Mahon said, "and they don't want to take unnecessary risks with national defense this year. This seems to be the mood of many."

Carter, who has vowed to cut the defense budget, may have helped pave the way for the increase with his hard-hitting defense speech last week in Winston-Salem, N.C.

In that speech, the president, decried the "massive forces" that the Soviet Union is building and pledged that "we will not allow any other nation to gain military superiority over us." He mentioned that he might order the MX nuclear blockbuster missile into production if the Soviet buildup continues unabated. The Pentagon currently has the MX at a medium development pace, requesting $158.2 million for the weapon this year compared to $134.4 million last year.

Mahon said the MX was "one of the candidates" for the expected congressional additions. He said adding money for another Nimitz-class nuclear aircraft carrier, as the House Armed Services Committee is advocating, "would set you back big." Carter did not seek any money this year for a nuclear carrier.

Charles W. Ducan Jr., deputy secretary of defense, said in a meeting with reporters yesterday that $126 billion "is an appropriate number" and one that takes full account of the Soviet military buildup Carter decided in his speech last week.

Duncan characterized the president's speech as "a strong reaffirmation of existing U.S. defense policies." Ducan added that "I don't see anything this year" in the way of Pentagon requests for extra money and that "clearly, we have not" asked Congress to add to the fiscal 79 defense budget now before it.

While running for President, Carter said he could cut Pentagon spending by $5 billion to $7 billion every year. The Carter administration contends that the fiscal 1979 request for $126 billion fulfills that pledge because it is that much lower than President Ford would have requested under past projections made by the Republican administration.

The House Armed Services Committee wrote the House Budget Committee last week that it believed $2.6 billion would be added to the president's fiscal 1979 defense budget. Additions the committee intends to recommend to the House include $2.3 billion for a Nimitz-class carrier and $2.1 billion for a nuclear cruiser.

To help offset these and other increases, the committee served notice that it intends to lp $912 million from the Pentagon's request for money for the Navy's Trident missile submarine.

The House Armed Services Committee, in its letter estimating the size of the fiscal 1979 defense budget, said its $2.6 billion in additional budget authority would equate to a $441 million increase in spending. The extra money is needed, said the committee in recommending the addition on a 25-to-5 vote to combat the Soviet buildup.

The Senate Armed Services and Appropriations committees recommended an addition of $1.7 billion to the president's defense budget request. The Senate Armed Services Committee said the additions were required "in light of the continuing long-term quantitative and qualitative expansion of power and military capability of the Soviet Union." That committee said it was especially concerned about strategic programs in the Navy's shipbuilding budget.