National security adviser Frank C. Carlucci predicted yesterday that the "98 percent" complete U.S.-Soviet treaty banning U.S. medium-range and shorter-range nuclear missiles will win Senate ratification, but a leading Senate Democrat warned that legislative amendments could have the effect of killing it.

"We think it will be ratified," said Carlucci of the intermediate-range nuclear forces (INF) treaty that President Reagan and Soviet leader Mikhail Gorbachev are scheduled to sign at their summit meeting here beginning Dec. 7.

Speaking on NBC News' "Meet the Press," Carlucci said the verification arrangement, which has been a major point of senatorial questioning, will be "the most intrusive in the history of arms control, and I think the senators will be pleased when they see it."

Senate Majority Whip Alan Cranston (D-Calif.), who said he expects to support the treaty, warned that conservative Republicans allied with other senators could doom the pact by attaching unacceptable reservations or amendments about matters such as Afghanistan, Jewish emigration or compliance with other treaties.

"If Ronald Reagan can't get this kind of modest treaty through the Senate, I don't know when any American president will be able to negotiate successfully with the Soviet Union," Cranston said. "That could doom us to a very dangerous, escalating, costly arms race, in terms of many, many years."

Senate Minority Leader Robert J. Dole (R-Kan.), who said he hopes to be on Reagan's side in favor of the treaty, defended the Senate's prerogatives in legislating on the subject.

"We have a role to play; it ought to be a constructive role," Dole said. "The Senate is not just to rubber-stamp treaties. We are supposed to go through the process, have the hearings, improve them where we can, and we're going to be an active participant in that."

The senators expressed uncertainty over whether the United States and Soviet Union will be able to complete a more sweeping and important treaty slashing strategic -- or long-range -- offensive weapons by 50 percent in the near future. A joint statement Friday pledged Reagan and Gorbachev to work toward the signing of such a treaty in Moscow in the first half of next year.

"I hope they don't rush to judgment" on a strategic arms accord, said Dole. Cranston said the Senate would have to move swiftly next year on ratifying the INF accord if a strategic arms pact is to be completed.

Assistant Secretary of State Rozanne L. Ridgway, who was interviewed on ABC News' "This Week With David Brinkley," expressed optimism on the basis of recent U.S.-Soviet discussions that the longstanding dispute over defense in space can be resolved. This issue has been tied by the Soviets to a strategic arms accord.

The Soviet position on Reagan's Strategic Defense Initiative (SDI) has been changing, Ridgway said. "In time we'll find the basis for going forward with the program, and they'll admit it," Ridgway said.

Carlucci also emphasized what he described as shifts in the Soviet posture toward SDI. An interesting aspect of last Friday's talks with Soviet Foreign Minister Eduard Shevardnadze, according to Carlucci, who participated, is that "the Soviets are discussing strategic stability. They are no longer talking about killing SDI."

Both Carlucci and Ridgway said that despite confirmed reports of dissension among Kremlin leaders at a recent meeting of the Communist Party Central Committee, they believe Gorbachev remains firmly in charge.