SYDNEY — CONGRESSMAN Stephen Solarz (D) of New York says that the United States may have to pull its naval forces back to Guam if the Philippine Senate rejects the US treaty on the continued use of the Subic Bay Naval Base."Guam is mostly likely our fallback position," says Mr. Solarz, chairman of the Asian and Pacific Affairs Subcommittee of the House Foreign Relations Committee. If the Philippine Senate rejects the US Treaty proposal, Solarz says, "I think there is a good chance we are going to be expanding our military presence on Guam." A majority of Philippine senators endorsed a resolution yesterday to reject the treaty. A final vote is expected before Sept. 16, when the current treaty expires. The Navy today has 7,000 personnel and 6,000 dependents on Guam. Five ships and three air squadrons are based there. Since the eruption of the Philippine volcano Mt. Pinatupo, the Navy has used Guam for training Navy Seals, Marines, and other small units which normally would have used the Philippines. However, Lt. Dave Wray, a Navy spokesman, says any major buildup would have to be phased in. "We don't have the infrastructure for a sudden increase." An increased naval presence on Guam, a US territory, would not be popular on the island. Congressman Ben Blaz (R) of Guam and Governor Joseph Ada are both actively trying to move the US Navy's present air operations from Agana to Andersen Air Force Base on the northern part of the island. Guam wants to use the US Naval Station at Agana to expand its airport. Guam officials say the island would not be able to take the extra personnel of an expanded naval presence. They believe the infrastructure is already stretched tight. A recent surge in Japanese tourism has helped ease unemployment, but has put stress on the island's road system and its airport. The US military used the Andersen Base earlier this year when it evacuated 18,000 families from Clark Air Base after the Mt. Pinatubo eruptions. Only about 200 airmen, however, were permanently transferred to Guam from Clark. "There was no significant economic impact on Guam," says Vernon Perez, a spokesman for Mr. Blaz, a nonvoting delegate to the US Congress. In an interview, however, Blaz said Guam would not be a good substitute for Subic Bay. Guam's harbor is not deep enough, he says, and there is not enough labor on Guam to work for the Navy. Subic Bay hires 4,000 local people to work at a ship repair yard. In addition, Blaz says the Navy should not make the decision without consulting the people of Guam, which is considering changing its political status to a commonwealth. "The days of moving in or out at the whim of some Navy commander are gone." If the Navy must leave Subic Bay, Blaz suggests that Guam be one of several areas, such as Japan and Singapore, to get the US Naval Station. Solarz believes any Soviet threat to the region is now diminished. "The Soviet Union will be spending its time on domestic issues rather than trying to project power in the Pacific," the congressman says. He does not believe this will allow the US to reduce its forces in the region, however. The major US military presence in the region is in Korea, Japan, and the Philippines. The US troops in Korea help to counter the North Korean forces, which are not likely to be reduced in the short run. The forces in Japan represent a "strategic reserve" for the region. And the military in the Philippines helps safeguard strategic sea lanes. "The US will always be a Pacific power," Solarz says. The Soviets, however, will see their influence wane, he says. Among the casualties will be the large US-built Soviet Naval Base in Cam Ranh Bay, Vietnam.