President Reagan's national security affairs adviser, William P. Clark, yesterday made a strong pitch for keeping open the option of temporarily deploying new MX missiles in older missile silos.

He also declared that modernization of the nation's strategic nuclear forces "will receive first priority in our efforts to rebuild the military capabilities of the United States."

In according top priority to building American nuclear strength, Clark may have been seeking to beef up the American negotiating position for new strategic arms reduction talks (START) with Moscow that the Reagan administration has proposed get started in Geneva late next month.

But Clark's statement appeared to conflict with an assessment given a few weeks ago to the Senate Foreign Relations Committee by Defense Secretary Caspar W. Weinberger.

At the time, Weinberger was emphasizing that, contrary to some "totally erroneous reports," there had been no slackening of American concern for and interest in the North Atlantic alliance.

But he went on to say: "Let me dispel another misconception. The Reagan administration is not placing greater emphasis on nuclear weapons. We are, in fact, stressing the revitalization of our conventional capabilities. Only 15 percent of our budget is devoted to nuclear forces . . . and that will actually decline. The remaining 85 percent is devoted to non-nuclear forces."

In his pitch yesterday to retain the option of putting MX in "a limited number of Minuteman silos," Clark was also challenging Congress, which has cut $1.5 billion from the budget for the MX because Congress opposes placing the missiles in existing silos and wants the Pentagon and White House to come up with a better answer.

Clark confirmed that the president has "provided some guidance" to the Pentagon in recent days on priorities that Reagan "wished accorded to various basing and defense schemes" for the MX so that congressional demands for a decision by this fall on a permanent scheme could be met.

The most favored permanent plan at the moment is called "Dense Pack," which involves building tight clusters of missile silos for the MX and which also could mean a total break with the unratified SALT II accord with Moscow, which does not permit building new silos.

But Clark said that Reagan also made it clear that until a permanent home is found for the missiles the Minuteman basing plan is an essential hedge. "The MX program is too important to allow the risk of technical, environmental or arms control debates to delay the introduction of the missile into the force," he said.

Clark said the MX, even in the old silos, would be a clear incentive for Moscow to negotiate, and that even in such silos the "MX gains in survivability." This claim was not explained.

The speech at Georgetown University's Center for Strategic and International Studies was Clark's first major public statement since becoming Reagan's special assistant for national security in January.

The reason for the speech, Clark said, was to report in general terms on a just-completed review of national security policy. But the speech appeared aimed in large part at emphasizing the image of a president heavily involved in these matters.

"The president's involvement in this study is a good example of how he involves himself in national security affairs," Clark said early in the speech. "In this particular security review, the president played an extraordinarily active role," Clark added. And when the study was finished, "the study and the decisions were the president's," he continued.

The president "has already signed 35 national security decision directives, 19 of them this year, a pace that compares favorably with his predecessors'," Clark said of his boss.