Sen. Warren Rudman of New Hampshire is a pro-defense Republican who most of the time supports President Reagan. He championed the B1 bomber on the Senate floor, voted for two new nuclear aircraft carriers and favored hefty yearly increases in military spending, though not as much as the 10 percent after inflation that Reagan wants. But he says he is not sure he can go along with the president on the MX missile.

"I'm a fence-sitter on this one," he said.

Rudman's indecision shows what Reagan and his lieutenants will be up against over the next several weeks as they try to persuade Congress' political center to approve construction of the MX and deployment of 100 of the new intercontinental ballistics missiles in Minuteman silos.

"You don't get any quantum increase in deterrent" by building only 100 MXs, Rudman said on the basis of secret briefings given by Reagan's special advisory commission on nuclear arms and by Air Force officers who man Minuteman missile sites in North Dakota.

"To get that," Rudman said of the nuclear firepower needed to underscore for the Soviets that firing first would be suicidal, "you would have to deploy enough MXs to blanket not only their hard-target capability, but their command and control capability." President Carter got closer to that, Rudman said, by recommending deployment of 200 MX missiles.

If the question of whether to spend an additional $17 billion to build and deploy the MX hinged only on how much it would add to the U.S. nuclear deterrent, Rudman continued, "I'd vote against it. So now you've come to the second question, which is this whole new area of discussion that has grown up around here: the great marketing techniques" forming the rationale for deploying the MX in the American West and the Pershing II in West Germany and building a new stock of poison gas.

"People must stay up nights to come up with new marketing techniques," he said. NATO commander Gen. Bernard W. Rogers, the president's arms commission, "most of the State Department folks, people in the White House, they all say that we are being looked at by our allies in western Europe; that our allies in western Europe need to see American will to deploy a new strategic system.

" 'Now listen,' I say to myself, 'Why?'

"They say the Europeans want to see us deploying a new system to show the American people haven't changed in their will. We're asking the Europeans to allow us to deploy Pershing and cruise missiles on their soil. Do we have the will to deploy these" MXs on our soil?

"I don't know about this. I frankly make no judgment on the validity of that claim. I intend to talk to a lot of people both in and out of government to try to assess that in my own mind."

Rudman said that the second MX argument he is assessing is the administration's claim that going ahead with the missile will provide leverage for U.S. officials trying to negotiate reductions in nuclear arms deployed by both superpowers.

"There may be a heavier basis to this" than the one about demonstrating U.S. will to the Europeans, he said, but again he said he has not made up his mind.

He has rejected, however, the anti-MX argument that it makes no sense to offer the Soviets a choice target of the 10 nuclear warheads on the MX instead of the three on the Minuteman III by putting the MX in the Minuteman silos, which the Pentagon has said are vulnerable.

"I don't think the Russians will now say, 'Aha, we now have more of a first strike' incentive," he said. "I reject that." One of his reasons is that U.S. bombers could get off the ground as Soviet warheads bore down on MX missiles in the Minuteman silos.

Whether it is worth spending $17 billion more on the MX to get it deployed in the Minuteman silos by 1987 when the small, mobile, single-warhead missile that the MX commission also recommended for the future and that Rudman likes will be ready only four years later "is a tough question. I'm not sure it's worth it. That's what I'm weighing right now."

Rudman added that the cost and short life of the MX will not be a determining factor if he decides that foregoing MX deployment would have "adverse political effects, adverse negotiating effects . . . . I'm very disturbed" about that possibility. "I've got some people I want to talk to, some Europeans," he said, about the political consequences of not deploying the MX.

"I've already reached a conclusion that purely on strategic planning, I couldn't vote for this," Rudman said, "because it doesn't give us enough for the money we spend. It's a better offensive weapon, not a better deterrent. And there's a very distinct difference between those two, as I look at them."

Rudman said he has been in touch with doctors and leaders in the nuclear freeze movement as he ponders his MX decision. He said people in the movement are very sincere. Rudman blamed Reagan administration officials, but not the president, for arousing fears by making reckless statements about nuclear warfare.

"Asinine, that's the word I would give to" Defense Secretary Caspar W. Weinberger's statement about preparing for "protracted" nuclear war. Weinberger used that term last year issuing secret guidance to the military services.

"I spent about 15 months in combat as a company commander in Korea during the Korean War," Rudman said. "I know what one good artillery barrage will do, put on a small area of land. Anybody who talks about protracted nuclear war has never heard a shot fired in anger. There will be no protracted nuclear war. It'll be very short."

Rudman said that he has not yet felt much lobbying pressure on the MX. In his review of the troubled history of the MX, he said Carter's plan to rotate 200 missiles among 4,600 cement garages in Nevada and Utah was "probably the best basing mode of all." Its flaw, he said, was the possibility that the Soviets might build enough warheads to target every garage.