Democrats who expected arms control to be their issue in 1988 are instead bracing for the Reagan peace offensive that a U.S.-Soviet arms control treaty and a Washington summit will bring.

As the negotiations on an intermediate-range nuclear forces (INF) treaty reach their climactic phase this month, the political debate over this controversial accord is gradually shifting from Europe to Washington and is becoming entwined in the positioning for 1988's presidential election.

It is certainly no accident, as they say in Moscow, that the Soviet Union has chosen this exact moment to offer President Reagan a particularly difficult choice on the final shape of the agreement that would secure Reagan's place in diplomatic history.

The Russians are now dangling the possibility that they will also give up 100 SS20 missile warheads they had earlier insisted on keeping in Asia -- if the United States will acquiesce in a tacit phasing out of 72 Pershing IA rockets stationed in West Germany under nominal dual German-American control.

Such an agreement would mean that Reagan would have succeeded in ridding the world of the triple-warhead SS20. It would also remove serious verification problems that would be posed for the United States in letting the Russians keep 100 SS20s, their maintenance facilities and probably some limited production capability.

But West German Chancellor Helmut Kohl has made retention of the aging Pershing IAs a point of honor. Britain and France would also be extremely unhappy about the precedent implied by Washington throwing a "third-country" force into a U.S.-Soviet deal.

Under the terms of their original deployment, the IAs are scheduled to go out of service in 1989. A commitment not to replace them would probably result in the Soviets giving up the Asian SS20s, U.S. negotiators have been led to believe.

This is likely to be the last detail of the draft treaty to be settled when Secretary of State Shultz and Foreign Minister Shevardnadze meet in Washington later this month.

The treaty is close enough for Democrats on Capitol Hill to have begun describing it as a "modest and flawed agreement" that should not be allowed to overshadow six years of U.S.-Soviet deadlock under Reagan.

Presidential hopeful Sen. Joseph Biden predicts that voters will understand that the INF agreement is "a sideshow" that "does not address the real issues of strategic arms and of defensive systems."

But Biden acknowledges that he and other Democrats will almost certainly vote to ratify an arms treaty negotiated by Reagan and sent to the Senate.

"I would not have negotiated it that way," Biden said. But "it establishes a climate for the next president to make genuine progress" since Republican conservatives "will have cashed in their card of opposing arms control by supporting this questionable agreement."

The single most influential voice in the Senate on the treaty will be Sam Nunn, who is set to turn the debate into a more general confrontation over the administration's approach to the 1972 U.S.-Soviet Anti-Ballistic Missile treaty.

In an effort to reinterpret the ABM treaty to give the administration more latitude to pursue Reagan's Strategic Defense Initiative, the State Department has argued that testimony before the Senate on the 1972 treaty should be disregarded in favor of an examination of the lengthy negotiating history of the treaty.

"According to the State Department's position on ABM, we are not to really believe what they tell us up here, but have to rely on the negotiating history," Nunn said with more cold calculation than irony in his voice. "We will have to examine carloads of documents and information."

"We would not vote on the INF treaty under this cloud without a complete examination of the negotiating record," Nunn continued, "and that could take six months to a year." That is, the hearings could go well into the 1988 election campaign if he does not get some satisfaction on the ABM issue.

While planning to hoist the administration with its own petard, Nunn says of the INF treaty that "the probability is that it will be ratified." For one thing, the Georgia Democrat thinks that an INF agreement is likely to force NATO to do more in conventional defense.

However much it may complicate their efforts to paint Republicans as anti-arms control, the Democrats are not likely to oppose this agreement. As they note, it is flawed. But as they concede, it is workable and heads Soviet-American relations in the right direction.