With crucial congressional votes nearing on President Reagan's $100 billion plan to build the MX missile and B1 and Stealth bombers, the Pentagon's top technical experts are making the rounds on Capitol Hill attempting to focus attention on the weapons' overall capabilities rather than on the cost and specific controversies surrounding them.

Thus, Richard D. DeLauer, the Pentagon's top scientist, told a Senate Armed Services subcommittee this week that building a new bomber is important not only because it will be needed to penetrate steadily improving Soviet anti-aircraft defenses. How long the B1 will be able to get through those defenses is a controversial question.

Perhaps even more important, DeLauer said, is that the B1 can take off much faster and on shorter runways than B52 bombers and will be better designed to withstand the effects of nearby atomic explosions.

Therefore, he said, the B1 should be less vulnerable to being wiped out in a Soviet missile attack on U.S. bomber bases, reducing Moscow's incentive for a first strike.

If war should come, DeLauer argued that an improved bomber is needed because it can carry a much larger atomic punch than most missiles to knock out particularly well-protected targets deep underground and because it can survive long enough to give U.S. commanders time to locate and strike mobile Soviet targets.

In this view, it is a better bet to have bombers search for and find targets such as mobile enemy missiles or command posts than to retarget quickly a missile that cannot change course after launch.

While the Pentagon claims the B1 program will cost $20.5 billion without inflation and $28 billion with inflation, and the Congressional Budget Office estimates $39.8 billion with inflation, DeLauer argues that the B1 and the more futuristic Stealth radar-evading bomber will force the Soviets to spend huge sums on defensive weapons "that cannot be used to attack the United States and its allies."

This idea of forcing the Soviets into spending money on schemes to defend their weapons or make them more survivable, as opposed to expanding their offensive forces, is central to the Pentagon's argument for going ahead with the new silo-busting MX intercontinental ballistic missile.

As DeLauer told the subcommittee, the hope is to force Moscow "to focus future Soviet ICBM investment on survivability rather than modernization or expansion."

Although there is considerable debate in Congress about whether to build the B1, Stealth or both, and attempts to kill the B1 are certain in the coming weeks, few disagree that some new bomber is necessary.

Capitol Hill also seems to agree generally that a new missile, such as the MX, is needed. But the administration has not presented a convincing case to many lawmakers that it has any real idea what to do with these weapons.

"The American people are wondering when we are going to get our act together on MX," Sen. Henry M. Jackson (D-Wash.), told DeLauer. "This has been studied and reviewed and studied, and now we come to a totally inconclusive situation" that is "hurting" the U.S. defense effort and that of its allies, Jackson said.

He was referring to the administration decision to put the first 36 or 40 missiles, of 100 currently planned for deployment, in existing Titan or Minuteman silos that are to be beefed up with more concrete and steel to enable them theoretically to resist pressures as great as 5,000 pounds per square inch from nearby atomic explosions.

It will be decided in 1984 whether to house the rest of the missile force in more permanent locations such as new, extremely deep underground silos, which DeLauer said he feels will be the most survivable choice.

They might be placed aboard new missile-carrying airplanes that can stay aloft for days, a concept that Defense Secretary Caspar W. Weinberger seems to favor and that is receiving the major share of Pentagon research money. A third possibility is trying to defend MX land bases with antimissile missiles.

DeLauer says it will cost $21.6 billion to deploy the first 36 MX missiles in old Titan silos, with about $7.8 billion of that for the basing work. According to reports yesterday from Kansas, one of three states where Titans are based, Weinberger has informed local congressmen that the first Titans will be removed early next year at the rate of one per month to make room for the MX.

DeLauer argued that because the MX, which will make its first flight test early in 1983, will be ready in 1986, long before other basing schemes, it would be foolish to "put it in a warehouse." Better to put it in Titan silos temporarily and let Moscow know that "their silo-based forces would no longer be able to threaten U.S. ICBMs with impunity," he said.

But because even these improved Titan silos would be vulnerable to attack, critics argue that putting the MX in them only adds to the threat of atomic warfare because both sides would be tempted to fire first, or fire while an attack is on the way, rather than lose their major weapons.

DeLauer answered this way: "There is no nation rich or foolish enough to spend $5 billion to $7 billion on the survivability of a system unless it intended for that system to ride out an attack. We are not changing our consistently held view that launch under attack is a good capability . . . but a dangerously destabilizing strategy to depend on. We do not, and we will not depend on it."

To those who suggest that the Soviets might now adopt such a strategy, DeLauer said: "That suggests that the Soviets are not as rational as we, and I reject that view. I believe my Soviet counterpart can see the flaws and related potential for accidental catastrophe as clearly as I, and he will have no part of it.

"No," he said in what really is a reflection of what the Pentagon hopes will happen, "I think he the Soviets will select mobility, or active defense, or negotiated reductions . . . as his response, and we would welcome any of those as being stabilizing because of their defensive nature."