Defense Secretary Caspar W. Weinberger yesterday vigorously renewed his argument for increased military spending, saying that lower budgets would doom arms control and hearten the "treacherous forces" that oppose democracy.

Weinberger said the Soviet Union has now deployed 45 SS25 nuclear missiles, 18 more than he reported in December, which he said violates the SALT II arms accord. Only a continued U.S. buildup, he said, will discourage the Soviets from further violations and persuade them to negotiate seriously.

"Only by sustaining our defense program can we maintain the incentives needed to ensure Soviet agreement and compliance," Weinberger said in a speech to the Carnegie Endowment for International Peace. "All this should be obvious. That it is not suggests some basic confusion in the United States about how and why the Soviets negotiate."

Much of his speech appeared aimed at Congress, which appears likely to cut the arms budget to help reduce massive budget deficits.

"Unless we are granted sufficient means to exercise our responsibilities and fulfill our commitments, the door will be open to the most treacherous forces in the world today," he said. "Still, there are those who say we can cut the defense budget and suffer no ill effects."

Many politicians have said that the Gramm-Rudman-Hollings budget-balancing act, which President Reagan endorsed and signed, will force severe cuts in arms spending during the next five years. But Weinberger argued that the program of steadily increasing military budgets that the administration charted in 1981 should not be abandoned.

U.S. officials say that the SS25 intercontinental missile violates SALT II, which was never ratified but which both sides have pledged to uphold, because the treaty allows each side only one new missile and the Soviets already have developed the SS24. The Soviets maintain that the SS25, a mobile missile being deployed in horizontal garages, is a variation of an existing missile and so legal under the treaty.

The Soviets also have retired more of the older SS11s, according to congressional sources, than the number of SS25s deployed so far, thereby keeping the force within the totals permitted under SALT II.

In a December speech in London, Weinberger said the Soviets had deployed 27 SS25s. His estimate of 45 yesterday is "conservative," he said, "since we expect additional deployments in the very near future."

Weinberger said that if the Soviets are now considering reducing their arsenal as part of a new negotiated treaty, it is "not out of good will, nor out of some hope for stability, [but] rather as the price for controlling and reducing American strategic weapons.