The achievements of President Reagan and Soviet leader Mikhail Gorbachev in securing one nuclear arms-reduction accord and moving toward another have set the stage for a vital national-security debate that could determine the relevance of the Republican Party.

During the next six months, this debate will be played out in the Senate when the Intermediate-Range Nuclear Forces (INF) Treaty comes up for ratification. A more emotional version of this debate will occur in the GOP presidential primaries as Vice President Bush defends the treaty against assorted foes on his political right.

The debate goes beyond the treaty, which marginally reduces the risks of nuclear war and is a good deal for both sides. It also goes beyond "trusting the Russians," as the question is often put, for few Americans of any persuasion would trust the Soviets to honor a treaty that could not be verified.

The division within the GOP is really an argument between those who believe that international cooperation to reduce nuclear arsenals is essential and those who believe that the Soviet system is so abhorrent and unchangeable that the United States must maintain unmistakable nuclear superiority.

This quarrel is the residue, far removed, of the pre-World War II debate between "internationalists," who sided with the British and later with the Soviets against Nazi Germany, and anticommunist "isolationalists" who believed that the United States, protected by two oceans, should trust no nation but itself.

Republicans resolved this debate in 1940 by unexpectedly nominating Wendell L. Willkie for president. Now almost forgotten, Willkie was a captivating internationalist with the courage to defy what was then the conventional isolationist wisdom of his party's leadership.

President Franklin D. Roosevelt, though worried that Willkie would be a formidable opponent, called his nomination a "godsend." As Willkie biographer Steve Neal put it, "Almost overnight, Willkie moved the Republican Party out of its hidebound isolationism and sent a message to the world that Americans stood together against Axis aggression."

Largely because of Willkie, who broke with his party's leaders on the issue, Roosevelt won narrow passage of the Selective Service Act in the middle of the election campaign. With Willkie's help, FDR gained bipartisan backing after the election for the Lend Lease Act, which provided military aid to Britain and eventually the Soviets.

American internationalists in 1940 were challenged to build a coalition against Nazi Germany while U.S. allies still had armies in the field. The internationalist test today is to preserve the security of the alliance while forging arms accords that reduce the risk of nuclear conflagration.

When Willkie was nominated, Reagan was a 39-year-old Hollywood film actor and New Deal Democrat who voted for Roosevelt, the greatest internationalist of them all. As the years passed, Reagan became an outspoken Republican conservative. His anti-Soviet credentials were beyond reproach, but he never lost his internationalist moorings. Last week, he vied with Gorbachev in recalling the heady days of the World War II alliance when Americans and Russians fought on the same side.

History has now chosen Bush to carry the internationalist banner. He is vastly different in personality and background from Willkie, a charismatic businessman who never held public office. Bush, the "resume candidate" in the gibes of his opponents, seems to have held every office except the one that he seeks. He has rarely been accused of being charismatic.

But Bush, like Willkie, is riding a historical tide. He understands, as does the Republican rank-and-file, that an unending arms race increases the danger of nuclear war. He knows that nuclear arms reduction is essential.

The nomination of Willkie did not destroy isolationist sentiment in America. Instead, it confined this sentiment to the fringe and enabled FDR to proceed with a shared vision of international security. If both parties in 1988 nominate presidential candidates committed to nuclear arms reduction, today's great debate could also have a successful bipartisan resolution.

Dialogue of the Week: During the INF Treaty-signing last Tuesday, Reagan used the Russian words for "trust but verify," and Gorbachev interjected, "You repeat that at every meeting."

"I like it," Reagan replied.