Three candidates for the 1988 Democratic presidential nomination, making initial major foreign policy speeches of their campaigns and in agreement on major points, have called for a 50 percent reduction in U.S. and Soviet Union long-range nuclear missile forces, denounced President Reagan's Strategic Defense Initiative (SDI) and advocated putting more emphasis on conventional military forces.

Missouri Rep. Richard A. Gephardt and Massachusetts Gov. Michael S. Dukakis gave their speeches here yesterday, while former Arizona governor Bruce Babbitt delivered his Thursday night in Iowa City.

While each candidate has years of public service experience, each has concentrated primarily on domestic affairs and is just beginning to outline views on foreign policy and defense.

All three listed the prevention of nuclear war as the top priority of U.S. foreign policy and urged aggressive arms-control negotiations with the Soviet Union. They agreed, in Dukakis' words, that "our goal should be to prevent the use of a single nuclear weapon, whether strategic or battlefield . . . by the superpowers, by a regional power or by terrorists."

Gephardt and Dukakis noted that Soviet leader Mikhail Gorbachev offers the United States an opportunity and a challenge.

Gephardt described Gorbachev as "a different leader, speaking in different tones and with remarkable sensitivity to Western public opinion . . . we have to test whether it is real and how far it may reach."

Aware of the Democrats' past vulnerability on appearing to be soft on the Soviets, Gephardt emphasized the need to back negotiations with moral and military strength.

"I stand before you as a member of the party that has won the White House only once in the last five elections -- a party that has been equated, fairly or unfairly, with soft-headedness and faint-heartedness," he said. "The person Democrats nominate to carry our standard next year must be someone who has the record of resolve . . . to reclaim our rightful position as the party of a strong America."

All three were critical of Reagan's buildup of U.S. strategic, or long-range, missile forces and accused him of measuring national security in dollars spent rather than in the strength of the defense forces.

"Almost $2 trillion later, the nuclear balance of terror is unchanged," Dukakis said. "We do not have the resources to continue throwing money at the Pentagon."

He said the country needs "new leadership that will insist on discipline and accountability . . . that will give us some stability in defense spending . . . that will resist sacrificing military preparedness in order to protect big weapons projects." He promised that in a Dukakis administration, "it will be the commander in chief -- not an admiral or a lieutenant colonel -- who will be accountable for the national security operations and policies of the United States."

Gephardt argued that the next president should outline a comprehensive global defense strategy and "then buy only those weapons needed to support it," rather than basing defense spending on an arbitrary goal such as a 600-ship Navy.

A crucial element of this is shifting some of the responsibility to American NATO allies in Europe and Japan, he said.

"They must join in sharing the responsibilities as well as the privileges of the free world," Gephardt said. "We must ask them to bear a greater part of the military burden . . . . Something is wrong when the United States is borrowing money from Japan to pay for ships to protect Persian Gulf oil on its way to Japan."

Babbitt denounced Reagan's SDI, also known as the "Star Wars" program, as "the most reckless act of his presidency" that might be a "perfect, leakproof, foolproof shield against arms control" instead of Soviet nuclear attack.

"Star Wars greatly increases the risk of nuclear war because it provides powerful incentives to strike first," he said. " . . . To planners on either side, in the midst of some spiraling crisis, it might begin to look more rational."

All three advocated a new antiterrorism strategy, which Dukakis described as "a law enforcement" rather than a political issue.

Gephardt said that if elected he would send his secretary of state or some other top administration official to the Middle East "full time to do the hard work necessary to getting a comprehensive peace plan there." He said he would try to get "Jordan or some other nation to recognize Israel and its borders and its right to survival" as an important part of the process.

Dukakis told his audience that he had had breakfast in Boston with President Oscar Arias of Costa Rica and said the United States "should support his peace plan enthusiastically. It offers the best hope of ending the fiasco of America's illegal policy in Central America."