Rejecting calls from Republican lawmakers to overhaul the Anti-Ballistic Missile (ABM) Treaty all at once, President Clinton has decided to ask Russia to agree initially to relatively modest changes in the 27-year-old agreement, administration officials said yesterday.

The decision follows months of debate within the administration over whether to seek wholesale changes in the treaty immediately or take a two-step approach as the United States attempts to build a nationwide defense against missiles.

Administration officials said the gradual approach would improve the chances of reaching an agreement before presidential elections next year in both countries.

The first set of changes sought by the administration would permit the United States to place 100 interceptor missiles in Alaska, which is the Pentagon's latest plan for defending the country against, at a bare minimum, a few incoming warheads from a state such as North Korea, Iraq or Iran.

As the missile threat is perceived to grow and as U.S. technologies improve, officials said, the United States would seek further treaty amendments to permit more than 200 interceptors, at least two launching sites, advances in radar and the use of space-based sensors.

But congressional Republicans attacked the strategy, accusing the administration of squandering an opportunity to alter the treaty substantially now and arguing that the phased approach would only prolong tensions with Russia. They said that Moscow, which has long opposed U.S. defenses against long-range missile attack, likely would reject even the limited proposal for modifications. They also predicted trouble in Congress.

"The administration is very clear on what would be acceptable, and the minimalist approach is not acceptable," one senior Senate Republican staff member said.

Deputy Secretary of State Strobe Talbott, the administration's top Russia expert, flew to Moscow yesterday to begin discussions on the phased negotiation plan. Other high-level exchanges are due next week when Defense Secretary William S. Cohen visits Moscow and Russia's deputy foreign minister, Georgi Mamedov, comes to Washington.

European allies also were being informed of the U.S. plan this week, officials said. Concern that the Europeans might take issue with a more aggressive U.S. approach was a major factor in the decision to proceed in steps, according to officials involved in the decision.

"We also have to get the concurrence of our allies in order to make an effective anti-missile system," Cohen said in an interview yesterday. "They still look on the ABM Treaty as being one of the stabilizing factors in the relationship with Russia. It's important for us to proceed in a responsible fashion."

Some senior defense officials reportedly argued within the administration for a broader negotiation with Moscow. But Cohen insisted that he and Clinton's other top national security aides were unanimous in their support for the phased approach.

"This first phase will give us the kind of protection we'll need for the immediate missile threat," he said.

At the same time, Cohen stressed that the Russians would be told of longer-term U.S. plans to expand the anti-missile system and to seek further treaty changes in a second set of negotiations at a future date.

While Clinton has yet to approve the deployment of any national missile defense system, he has come closer in the past year to a decision to build one under pressure from Republican lawmakers and amid evidence that a growing number of nations are acquiring ballistic missiles.

In January, Clinton pledged $6.6 billion over the next six years for construction of a network of radars and interceptor missiles. The administration also announced then that it would ask Russia to renegotiate the ABM Treaty to permit a limited system of missile defenses. Months of debate ensued over how to structure the talks.

The ABM Treaty, signed by President Richard Nixon and Leonid Brezhnev, strictly limits the number, type and placement of missiles that Washington or Moscow can deploy to shoot down incoming missiles. Its fundamental premise, which held throughout the Cold War, was that limiting missile defense would discourage development of more offensive nuclear weapons and make each side confident that it had a credible deterrent against attack.

Despite mounting calls by Republican lawmakers to scrap the treaty as a Cold War relic, the Clinton administration has opted to preserve it as a cornerstone of nuclear strategy, essential to avoiding a new nuclear arms race.

U.S. officials have urged the Russians to view the deployment of a limited U.S. antimissile system not as a threat to the strategic balance between the two nations, but rather as a weapon against attack from "rogue states." But the Russians regard the scaled-down plan as a forerunner to reviving the more ambitious "Star Wars" system proposed by President Ronald Reagan in 1983--a space-based shield to protect the entire country from thousands of incoming nuclear missiles.

To entice Moscow into a deal, U.S. officials plan on trying to couple a new ABM Treaty agreement with a new strategic arms reduction treaty, START III, that the Russians want and that could reduce each side's nuclear arsenal to 1,500 warheads from about 6,000.

There are still enormous technological and financial obstacles to a national missile defense system. Chief among them: The Pentagon has yet to prove it can build a system that works. Clinton faces a decision next summer over whether to authorize deployment, but many experts predict the deadline will slip because of testing delays.

To permit the initial system that the Pentagon envisions, U.S. officials said they need agreement from Russia to designate a new site, substituting Alaska in place of Grand Forks, N.D., which was picked in the mid-1970s when the United States briefly activated an antimissile system.

The treaty allows a single site for protecting either a set of strategic missiles, as was the case in North Dakota, or a nation's capital. But it specifically bans an antimissile system to protect all national territory. This prohibition also will have to be renegotiated, officials said.

Other similarly contentious provisions restricting radar locations and basing as well as the use of space-based sensors would be postponed under the U.S. plan until a later phase of talks.