Defense Secretary Harold Brown took up the baton today in the administration's campaign to attract conservative opinion both in the Senate and the nation for a new strategic arms limitation treaty with the Soviet Union.

Speaking at a joint session of the Council on Foreign Relations and Foreign Policy Association here, Brown emphasized that SALT II "will have to be accompanied by substantial U.S. defense programs in the strategic field," greatly increased above existing spending levels.

Brown's speech following by a day an address by presidential assistant Zbigniew Brzezinski on the political need for SALT in the Soviet-American relationship, was intended as the administration's security justification for the long-awaited treaty.

Approval of the treaty by negotiators from both sides now awaits only a Soviet response on several difficult points regarding U.S. verification of Soviet compliance.

Although the administration is going on the assumption that final problems will be overcome quickly, Brown said today that it would be "a matter of some moths"-possibly "the remainder of this political year"-before Senate approval could be concluded.

High administration sources say that despite the concern expressed by some Senate liberals that the treaty does not go far enough in curbing the arms race, they do not expect them to vote against approval.

The administration's campaign for votes, they say, will be aimed mainly at reasurring conservatives that the treaty does not in any way jeopardize national security.

In his speech today, Brown emphasized that "despite Soviet military accomplishments, the Soviet Union does not now enjoy a military advantage in nuclear terms over the United States; It is not in a position to exploit its strategic weapons or embark on a course that may lead to the use of nuclear weapons without themselves encountering unacceptable risks."

Brown noted that the United States has been able to negotiate reductions already worked out in Vladivostok-to 2,250 strategic delivery vehicles and 1,200 missiles capable of firing multiple warheads. The treaty also imposes a new limit of 820 land-based missiles with multiple warheads-the force considered of most concern to U.S. strategic security.

"Without SALT II, the Soviet Union could have nearly one third more strategic systems than with the system," said Brown. "In my view, it is probable that without SALT II we would enter into an era of greater uncertainty-in both military and political terms-that would result in increased strategic forces on both sides."

During a question-and-answer period that followed, Brown was asked if the strategic arms talks had exerted pressures on both sides to continue rather than slow down the arms race. To that he replied: 'I'm not aware of any case where, because of SALT, some new system was developed or employed which wouldn't have been [in the absence of SALT]. I disagree completely that SALT has made things worse."

The defense chief stressed that the proposed new treaty will permit the United States to develop, test and deploy each of the three major new strategic systems designed to counter the Soviet threat in the 1980s. These are the Trident submarie, the MX landbased missile and cruise missile.

Figures issued by Pentagon officials this week on the comparative level of U.S. spending with and without SALT clearly suggest the limited nature of the agreements being concluded by the two countries. Without SALT, they said, the spending on strategic systems would be some $67 billion over the next five years. However, this spending figure will still be around $52 billion even with a SALT agreement.

Pentagon officials says that the cost of deploying a mobile, land-based MX missile capable of evading the threat of modernized Soviet land missiles in the 1980s would be around $20 billion. A mobile system employing airplanes would cost at least $30 billion, they add.

"By any reasonable standard, we have a credible deterrent today and intend to have one for the foreseeable future. We have, and will continue to have, survival forces capable of massive destruction of Soviet cities and industrial potential, even after an allout surprise attack. The rapid Soviet buildup in strategic forces over the past decade, as compared to our more modestly paced improvements in forces, should not obscure the basic power and credibility of our deterrent," Brown declared.

Administration officials, including Brown, believe the Senate approval process could hinge on Soviet assurances on treaty verification. The American side wants guarantees against Soviet coding of the telemetry returned by Soviet rocket tests.

At the same time, administration sources have acknowledged that the loss of U.S. monitoring stations in Iran has complicated the task of checking on Soviet compliance with the treaty.

Brown noted, however, that "not having a SALT agreement does not make verification easier." Sources said that contacts with friendly countries subsequent to the Iranian losses have led to optimism that backup systems can be installed.

Some administration officials have expressed concern that Senate critics of the treaty could attempt to amend the document. Commenting on this today, Brown said this would have the same effect as rejecting it since it would force both sides back to the negotiating table. "It would have tragic consequences for ourselves and the world." he added.