Defense Secretary Caspar W. Weinberger affirmed the "philosophical and moral superiority" of the United States over the Soviet Union yesterday and called on conservative politicians worldwide to help in "spotlighting the inherent evil" of communism in world forums.

Addressing conservative party leaders from 20 nations at the International Democrat Union gathering here, Weinberger recalled the "uproar" in 1983 over President Reagan's characterization of the Soviet Union as an "evil empire."

"What does it say about the current state of political culture in the West that so many in intellectual circles should deem such a remark inappropriate or perhaps a gaffe of some sort?" Weinberger said.

"It's not that the critics really argue that the characterization is untrue; I've heard very little of that. But it is that spotlighting the inherent evil of the coercive and tyrannical system of our communist adversaries is done so seldom that it strikes some people as being crass or even provocative."

Coming five days before 10th anniversary observations in Finland of the Helsinki Accords, agreements among 35 eastern and western nations on European security and cooperation, Weinberger's remarks appeared to serve notice that U.S. positions have not softened since Reagan's "evil empire" remark.

Weinberger attacked the growing use of "bad terminology" as discouraging conservative groups from plain speech. The word "superpowers," for example, "implies that the great divide in international affairs is somehow between the superpowers and all other nations, rather than between the democrats and the totalitarians," he said.

The term "arms race" implies that U.S. defense efforts are "empty of any broader significance than a sporting event," while "the use of words like 'guerrilla,' 'commando' and 'national liberation fighter' to refer to terrorists" is something that "tends to paralyze democratic politics" in responding to terrorism, he said.

Prime Minister Margaret Thatcher of Britain echoed Weinberger, warning against "a massive propaganda offensive aimed at the people of our countries" from the Soviet Union this autumn.

The Soviets will argue that there could be peace "if only the United States would give up the SDI Strategic Defense Initiative, or "Star Wars" proposal , if Great Britain would abandon its nuclear deterrents . . . only, in other words, if we accept the Soviet view and give up our own. That we will not do," she said to loud applause.

Under younger Soviet leaders, Thatcher continued, "the reality of the nature of communism has not changed, even if its image has been touched up."

Vice President Bush, standing in for Reagan during his convalescence from surgery, read a message from the president to the IDU. "The tide of history is a freedom tide," the message said, "and one reason for the change is the IDU."

Bush said Reagan was "strong, he's upbeat. The danger is, he's overdoing it a little bit, but what you see is what you get."

The two-year-old IDU seeks to promote cooperation among its 22 member parties from 20 nations. Japan, Australia and New Zealand are the only members outside Europe and North America.

In their joint communique, the party leaders included a surprisingly strong statement condemning apartheid policies of racial segregation in South Africa. It called on the Pretoria government to "open a national dialogue with all racial groups" to move toward "major reforms guaranteeing the equal participation of all South Africans in political life," and also called for implementation of a U.N. Security Council resolution demanding free elections in the South African-administered territory of Namibia.

IDU Vice Chairman Franz-Josef Strauss, head of West Germany's Christian Social Union party, called the demand "unrealistic."

The leaders also expressed "understanding" of Reagan's SDI proposal on grounds that "the pursuit of research into defense against ballistic nuclear weapons is justified in the interests of the free world."

And they denounced "the reneging of security obligations" by New Zealand, which has refused to allow port visits by U.S. ships that might be carrying nuclear material.