These are supposed to be doleful days for defense contractors. The military budget is being squeezed. New weapons programs are on the chopping block. And to top it off, the Reagan-Gorbachev summit that begins today is threatening to usher in a new era of arms control and detente.

Very bad for business.

So runs the conventional wisdom within the defense industry. But Peter E. Rupp, chairman of the Portland, Ore.-based Freightliner Corp., has different ideas.

Last week, Freightliner delivered to the Army the first of up to 1,028 new SEE tractors -- highly mobile vehicles that are so powerful they can bulldoze small mountains. Rupp said the SEE, which is assembled in Hampton, Va., is precisely the kind of weapon the Pentagon will need lots more of to strengthen NATO's conventional deterrent, a strategic imperative in light of the new U.S.-Soviet agreement to eliminate intermediate nuclear missiles from Europe.

"For God's sakes, I don't want to sound like the only reason I care about that treaty is because it gives us a chance to sell our products," Rupp said last week. "But the reality is that there will be a need to improve conventional forces.

"Like it or not, it {the arms control treaty} is a business opportunity," he added. "We're going to pursue that military business with a lot more vigor than we have in the past."

Freightliner's burgeoning military tractor business is one example of how many defense contractors -- ranging from big munitions makers to small consulting firms that surround the Capitol Beltway -- are hoping to exploit a dramatically altered defense environment. That new era, many industry analysts and officials agreed, will mean far less funding for strategic missile systems, tactical airplanes, nuclear aircraft carriers and other big-ticket weapons items.

But at the same time, they said, it opens up a wealth of market opportunities for other sectors of the industry: consulting firms that help the Pentagon monitor arms control agreements, high-tech research firms that devise ways to enhance existing weapons, and even some defense giants that are building cheaper and "smarter" weapons for conventional warfare.

"The main people who are going to profit are companies like ours that have diversified capabilities that can be moved quickly to whatever areas the nation needs to go," said Earle C. Williams, the president of BDM International Inc., the fast growing McLean-based consulting and professional services firm. "My theory is that, as we reduce our nuclear capability, we have no choice but to increase our conventional capability.

"And it so happens, most of the research and development work we've been involved in, including our work for SDI {Strategic Defense Initiative}, is directly applicable to new concepts in conventional weaponry. What this means is we expect to continue to grow."

Consider, said Williams, the electronic-magnetic rail run, an SDI-related weapon BDM is helping to develop for the Army. "That has direct applications to {conventional} antiarmor uses," Williams said.

"I think this is a terrific opportunity," added Ron Easley, chairman of System Planning Corp., a Rosslyn-based military consulting firm that specializes in nuclear strategy and arms control issues. "As a matter of fact, if I were going into business today, I couldn't imagine a better place to be."

As head of a $50 million a year "think tank" stacked with former officials of from Pentagon and intelligence communities, Easley is perhaps better situated than most to see immediate payoffs from the nuclear arms pact. Over the past year, System Planning has received about a $1 million a year for advising U.S. negotiators in Geneva on how to draft the treaty.

Now that it is about to be signed, Easley said he expect the government to turn to firms such as his to study such key issues as Soviet compliance and verification. Easley said he expects new contracts, not only tied to the treaty, but a whole range of related issues, such as restructuring the U.S. military posture in Europe.

In times like these, said Easley, "the impact is on the big guys -- it gets them out of the way and makes more money available for little guys, the smart little guys."

Yet at least one Washington area "big guy" in defense is still hoping to thrive in the new era -- Martin Marietta Corp., the Bethesda aerospace giant that ranked as the Pentagon's 10th largest contractor last year with about $2.9 billion in defense business.

"No question, these are harder times for defense contractors," said Norman Augustine, the firm's new chief executive, said in an interview last week. "But ... fortunately, not all defense contractors are alike. We've got the largest backlog we've ever had in corporate history, our sales are at a record level. And we think we can be among those few companies that will continue to prosper in this new environment."

At first glance, Martin Marietta might appear to be precisely the opposite -- a big aerospace company that could suffer greatly from arms control. Among its major product lines are Pershing missiles, which would be eliminated under the new treaty, and the MX or Peacekeeper missile, an intercontinental weapon that could be threatened if this week's summit leads -- as many hope -- to agreements on reductions in long-range strategic weapons.

But, Augustine said, "we've seen this environment coming for about four years so we've had a lot of time to get ready for it."

As a result, Marietta is unusually well diversified. Whereas rival General Dynamics Corp. has one program -- the F16 aircraft -- that accounts for nearly a third of its revenue and nearly half of its profits, Martin Marietta has as many as 10 programs that make up half of its sales.

More important, the company has been particularly well situated in devising what Augustine called "product improvements" -- enhancing the performance and capability of existing weapons through computers, enhanced imagery, laser technology and other means.

A prime example of Marietta's strength in these areas came last week when the firm won a new $608 million Air Force contract for continued production of its LANTIRN navigation and targeting system, an electro-optical system designed to enhance the ability of F15 and F16 aircraft to evade radar at high speeds or in darkness.

A potentially juicier plum came the same day when the Army picked Marietta, along with a Swiss firm, to build a new air missile defense system called ADATS to replace the Sergeant York gun. A high-tech conventional weapon, ADATS is designed to protect ground forces by shooting down attacking helicopters and low-flying jets.

Many defense analysts said ADATS is among the more significant new conventional warfare developments in some time, a weapon viewed as vital to strengthening NATO troops against the threat of a Warsaw pact invasion of West Germany. The Army has identified a need for as many as 562 ADATS units, which could provide from $4 billion to $6 billion in new business for Marietta and its Swiss partner, Oerlikon Buhrle.

As it turned out, the ADATS award was overshadowed last week when Martin Marietta lost out to rival Boeing Aerospace in a hard-fought competition to build the living quarters of NASA's space station.

But, Augustine noted, "we would expect a great deal larger business, in terms of dollars, from ADATS, than we would have had from the space station."