Sen. John Glenn (D-Ohio), potentially a key figure in the Senate debate on the new strategic arms limitation treaty (SALT), said yesterday that if he had to vote tomorrow, "I'm afraid I could not accept it [the treaty] withoug some qualifications."

Glenn said the Carter administration is "dangerously overestimating" current U.S. ability to monitor Soviet compliance with the SALT pact.

He suggested that the treaty should be delayed until new intelligence systems are in place that can guarantee sufficient monitoring capability.

"Where I part company with the administration," Glenn said, "is in its willingness to sign the [SALT] treaty now, even before we know for sure how well these prospective [monitoring] systems are going to work . . ."

Glenn made these remarks to the American Defense Preparedness Association in a speech that took him further away than ever from the Carter administration on SALT II.

The former astronaut and moderate Democrat has always been counted as a potential SALT supporter by headcounters for the administration.

Glenn has long said that the verifiability of the treaty was crucial for him. Since the loss of American listening posts in Iran that used to eavesdrop on Soviet missile tests, Glenn has expressed increasing skepticism that the new agreement can be minitored.

Discussing the Iranian listening posts yesterday, Glenn noted that "Secretary [of Defense Harold] Brown has said it will take about a year to replace these essential elements of our [monitoring] capability and I respect his judgment in these matters."

But, Glenn added, "We are gambling that plans still on the drawing board - ones that will not be available until some time in 1980 - work as predicted, and replace some of the capability we have lost. In this instance, where the stakes are so high, it is a big order to ask the U.S. Senate to accept only that prospective hope as a basis for verification."

Glenn was referring to administration plans to use U2 aircraft equipped with antennas and other eavesdropping devices to fly over Turkish airspace to monitor Soviet rocket tests from the Soviet Union's Central Asian test base. This is the central element in the administration's plans for compensating for the Iranian bases.

Glenn noted that "difficult political arrangements" would have to be worked out to put this plan into effect, a reference to the need for Turkish permission to use that nation's airspace. The Turkish government said this week that it would grant such permission provided the Soviet Union did not object, a potentially serious qualification.

"Frankly," Glenn said yesterday, "I am unwilling to support a treaty that we are unable to verify, particularly when the Soviets have given us ample evidence to question their peaceful intentions."

Without naming President Carter, Glenn sharply criticized Carter's recent comments on the possibility that SALT II might not win Senate approval. The President said defeat of SALT would invite the impression that "warmongers" dominated American policy, a statement that has provoked many senators.

Glenn said he doubted the SALT process would be brouhgt to an end if the Senate decided it wanted to substantially modify or even reject the new SALT pact.

"If the Soviets interpreted a failure to ratify SALT II as an end to talks, they do not want strategic arms limitations as much as I think they do," Glenn said. CAPTION: Picture, SEN. JOHN GLENN . . . differs with administration