Optimism Grows Over Prospects for Completing START Pact This Week : Arms control: Bush says the two sides are ‘99% there.’ Moscow shares that upbeat assessment.

U.S. officials are increasingly optimistic that President Bush and Soviet President Mikhail S. Gorbachev will overcome the last hurdle to final agreement on a strategic arms reduction treaty when they meet in London on Wednesday. And the upbeat assessment is shared by Moscow.

Concluding the START negotiations here--which White House officials stress remains a hope, not a fact--could offer Gorbachev a political plus back home to offset his expected failure to win major economic aid. The Soviet leader meets with the other summit partners after his session with Bush.

Secretary of State James A. Baker III and Soviet Foreign Minister Alexander A. Bessmertnykh announced Sunday that final agreement had been reached on all points in the treaty except the so-called throw-weight issue.

Bush later told reporters that while that technical issue remains to be resolved, the two sides are “99% of the way there.”

U.S. officials disagreed over just how difficult it would be to resolve that one issue--concerning the definition of missile throw-weight, or how much payload weight can be carried, or thrown, a given distance--before Bush sits down to lunch with Gorbachev at Winfield House, the U.S. ambassador’s residence here, on Wednesday. Nevertheless, the prevailing mood was one of optimism.

“Yes, a good feel now,” Bush told reporters Monday morning after speaking with Baker. “It’s encouraging--one point remaining, but an important one.”

Asked whether it would be possible to overcome the throw-weight issue in time to schedule a summit in Moscow by the end of the month--his oft-stated goal--Bush said: “I think we’re going to be talking about it--the experts are going to be talking about it--before I see President Gorbachev here.”

Similarly, Brent Scowcroft, Bush’s national security adviser, said in an interview Monday on NBC News’ “Today” show that it is “quite possible” that the matter could be settled by the time of the meeting.

“It is an important issue, but it is not beyond resolution,” Scowcroft said. “I am optimistic.”

Bush has said repeatedly that he will not schedule a summit in Moscow until the treaty is completed, clearly seeking to avoid the sort of wrangle that developed over definitions in the agreement to reduce conventional weapons in Europe, which was signed last November but then was subject to negotiations through the spring.

And not every U.S. official shared the general hopefulness.

One senior White House official, speaking on the condition of anonymity, said the remaining point in the START treaty, which would trim the U.S. and Soviet arsenals of long-range nuclear weapons by about one-third, is so important to the Soviets that “there is a good chance they won’t accept a compromise.”

He said chances are “no better than 50-50” that the pact will be completed in London.

In this official’s view, the United States is reluctant to rush the resolution, despite Bush’s desire to get to Moscow before beginning his vacation in Kennebunkport, Me., in early August. And Gorbachev has his own reasons for delay.

The Administration is well aware of the difficulties it could encounter gaining the required Senate confirmation of the treaty if it appears that the end game was completed hastily for political reasons. And Gorbachev does not want it to appear to his military that he is yielding on arms to gain Western economic assistance.

On the other hand, a U.S. arms control expert said that the tortuous nine-year talks are “down to something very confined and manageable.”

“As far as we’re concerned, there is light at the end of the tunnel,” he said.

Indeed, it was felt among those familiar with the negotiations’ apparently final stages that if the effort now goes awry, the failure would be an indication that either Bush or Gorbachev was using the final unresolved issue as a means to back out, for broader, political reasons that are being pressed on them by military leaders or conservative factions back home.

A White House official traveling with Bush to the summit said that among some in the Administration, there is a view that the treaty could be debated “for the next two years” without resolution.

But, he said, “there is another school of thought that this goes beyond being a technical issue to a political issue--one that needs a decision at the highest level.”

If it turns out to be the former, he said, “no one expects” a Moscow summit soon.

“If it’s the latter, you can have a summit announced on Wednesday,” he said, adding, “My gut tells me it’s the latter.”

Times staff writer Doyle McManus in Washington contributed to this story.

START Obstacle: Throw-Weight

The last issue blocking the START pact is the question of how much throw-weight would have to be added to an existing missile for it to be considered a new type of missile. Cheating would be defined as adding more warheads to the throw-weight than permitted under the treaty by both sides.

THROW-WEIGHT: The amount of payload a rocket can deliver at a specific range. Cosnsists of re-entry vehicle, targeting devices, penetration aids and warheads.