Pressure is building up on President Carter to deploy the Air Force's blockbuster missile, the MX, to offset the nuclear might lost through his cancellation of the B-1 bomber.

Paul H. Nitze, a leader of a group called Committee on the President Danger, said in a press conference yesterday that what to do about the MX "may well be the next important issue" in the arms debate.

Nitze and his allies contend that the MX, which would be kept mobile in deep trenches 10 miles long to make it hard to hit, is vital to maintain "rough equivalence" with the Soviet Union in nuclear weapons.

Opponents counter that deploying the MX would blow up chances for meaningful arms control and raise fears in Moscow that the United States was building a nuclear offense for striking first.

Carter himself has said he hoped neither the United Sates nor the Soviet Union would deploy mobile missiles woth intercontinental range like the MX. To back up this desire expressed in his first press conference, Carter subsequently reduced the Air Force budget for the MX from $295 million to $135 million, a cut of $160 million.

Air Force leaders told Congress in recent testimony that they planned to push that $135 million for fiscal 1973 to about $1 billion in fiscal 1979. How much money to give the Air Force for the MX is one of the questions now confronting Carter and Defense Secretary Harold Brown as they prepare the fiscal 1979 budget to be sent to Congress in January.

"Deployment of the MX system will yield great bargaining leverage," said the Committee on the Present Danger in a statement released yesterday on arms control negotiations.

Mobile MXs "should be highly survivable" so that some of them could knock out Soviet missiles not used in the first wave.

"MX could significantly reverse the unfavorable trends in the U.S. Soviet nuclear balance which would otherwise continue unchecked," said the committee in urging the Carter administration not to ban new or mobile intercontinental ballistic missiles or testing of them.

Nitze, in speaking for himself as an individual rather than the committee he helped organize, said carter should have kept the B-1 in production as well as build the MX.

Hiding mobile MX missiles in deep trenches would appear to violate the current accord between the United States and the Soviet Union which states that "each party undertakes not to use deliberate concealment measures which impede verification by national technical means of compliance" with the interim arms control agreement.

Carter said at his Feb. 8 news conference that if the Soviets "would agree to a cessation of the use or deployment of the mobile type missiles, that would be a very important point for us to join them in a mutual agreement." Nevertheless, the MX continues to move along the bureaucratic course.

The Pentagon's Defense Nuclear Agency recently informed Arizona lawmakers that it plans to set off a series of explosions in the state this fall to help determine how well an MX missile in a trench would survive a nuclear attack.

A defense Nuclear Agency spokesman said yesterday that in the yearlong testing program to be held near Lake Havasu City, Ariz., there will be as much as 600 tons of explosive set off at once to determine the effect on the ground.

"No adverse effects to the local population or to the natural environment will occur due to this program," Brig. Gen. Thomas E. Lacy, commander of the Defense Nuclear Agency field office at Kirtland Air Base, N.M., wrote Rep. Morris K. Udall (D-Ariz.) in a letter dated June 2.

Lt. Gen. Alton D. Slay, Air Force research chief, has said that the MX missiles buried in trenches 10 to 20 miles long would present more target area than available Soviet H-bombs could destroy. Each MX blockbuster missile moving along a trench could carry up to 12 H bombs of about 200 kilotons each with unprecedented accuracy. This would make the MX by far the most powerful nuclear missile in the U.S. arsenal.

One Air Force plan is to deploy 300 MX missiles by 1984 - an effort expected to cost about $20 billion, or close to the $24.6 billion the Pentagon estimated it would have cost to buy244 B-1 bombers.