"Arms control is the single most important issue in this campaign," President Carter told us in the Great Debate. He said that's what his daughter, Amy, told him. Both Defense Secretary Brown and National Security Adviser Zbigniew Brzezinski would have us look upon tomorrow's vote as a "referendum" on SALT II.

Smart politics?

That's partly it. The SALT treaty has become something of a test of right-thinking on War/Peace, a convenient club for pounding on Ronald Reagan as a trigger-happy arms-racer who could carelessly toss aside the world's last great hope for arms control. It's also good for taking the people's minds off inflation, the energy crisis, jobs.

But smart politics isn't the only reason Senate ratification of SALT II has been built up as the be-all-and-end-all of the Carter campaign. It isn't exactly a zinger, after all; it offends leading Democrats as well as Republicans and, for your average voter, comparing throw weights and counting warheads is pretty esoteric stuff.

Another much more intriguing -- and to me, more compelling -- reason emerged in the course of a long interview the other day with Secretary of State Edmund Muskie: President Carter, it turns out, conveyed through Muskie, early in the campaign, a firm promise to the Soviets that SALT II would be a centerpiece in his reelection fight.

"I told Soviet Foreign Minister Andrei Gromyko that our plan was to make this an issue on the campaign in order to build a constituency for it," Muskie explained. "It's an uphill fight, I told him, but I'm ready to do it, and the president's ready to do it and we're going to do it."

Why tell the Soviets? The explanation goes to the heart of Muskie's view of the significance of SALT II, not merely as an arms control measure but as a critical, determining element in the future course of U.S.-Soviet relations, across the board. It begins with the collapse of the SALT II ratification effort at the turn of this year.

And it rests on Muskie's belief that SALT II is not only "a very high priority for the Russians" but may also be a fundamental test, in their eyes, of how the United States views its relations with the Soviet Union -- how willng we are not just to control the arms race but also to seek at least some limited accommodation, to ease the strains, to reestablish something like, well, de'tente.

Muskie is not sure about all this. But he was at least impressed by some of the arguments Gromyko gave him in a meeting in Vienna last May. As the Soviets saw it, Gromyko told him, the United States had for all practical purposes "rejected" SALT II by not acting on it in late 1979. Their cynicism was hardened by what seemed to them to be a wholly manufactured "discovery" by the United States of the famous Russian brigade in Cuba.

It was compounded by congressional adoption of a new, five-year defense spending program calling for increases of several hundreds of billions of dollars. "You put those three things together," says Muskie, "and you can see how they might well reach that conclusion."

Muskie's not saying he believes it. But he can see how all this could have made it easier for the Soviets to contemplate risking a rupture with the United States by invading Afghanistan, on the theory that there was not all that much left in the relationship to rupture.

That's not to suggest that ratification of SALT II would necessarily hasten a Soviet withdrawl from Afghanistan. Muskie would not link the two in a bargaining sense. But he would hope that SALT's approval would strengthen those Soviet elements who might favor accelerating their withdrawl from Afghanistan in the interest of removing a serious source of East-West strain.

Muskie has conveyed to the Soviets that their presence in Afghanistan remains a serious "geographic" threat -- that it is as much a test of their intentions, in our eyes, as SALT II may be a test of American intentions, in theirs.

So SALT II's ratification, Muskie concedes, offers no guarantee of a general restoration of improved relations with the Soviets -- merely a hope, and an opportunity. But its rejection, he firmly believes, would strengthen Moscow's hard-liners and almost certainly make matters a good deal worse.

"It would be a very serious business to drop SALT II," he says. "The allies want it. The president wants it. And as far as I can see, the Russians want it. It may just be the one damn thing we can do to straighten out our relations. We've just got to make a credible fight."

Credible, that is, not just to American voters, but -- as Ed Muskie sees the SALT "referendum" -- to Andrei Gromyko as well.