The biggest missile in America's arsenal has grown so old and so tricky to handle that it may be more of a threat to its Air Force crews than to the Soviets. Congress has quietly asked the Pentagon to look into its safety problems.

At the same time, the 17-year-old Titan II is so big that, despite the fact that only 54 of them remain, they constitute a third of the land-based nuclear destructive power the United States could throw against an enemy.

Thus, at a time when critics accuse the administration of permitting the nation to fall behind the Soviet Union in deliverable destruction, decommissioning the Titan poses a political problem.

Interviews with officials at the White House, the Pentagon and on Capitol Hill indicate a feeling that the giant liquid-fueled intercontinental ballistic missile is today less a dangerous weapon than a dangerous symbol in the often irrational strategic arms race.

When they first became fully operational in 1962 these monster rockets -- each with a nine-megaton warhead, 750 times more powerful than the Hiroshima bomb -- had a role to play in threatening destruction to large Soviet and Chinese Communist cities.

Today, accidents involving the missiles's toxic liquid fuel occur regularly in a weapon system whose military mission seems to have all but faded away.

Last year, for example, two people were killed and 27 hospitalized in Kansas and Arkansas in two accidents associated with the highly toxic liquid propellants used in the Titan IIs.

Twenty more were sent to hospitals in smaller incidents at Titan silos going back to 1974, according to an Air Force report recently sent to Congress.

Earlier this year, the House approved an amendment by Rep. Dan R. Glickman (D-Kan.) that called for the secretary of the Air Force to review the need for and the physical condition of the Titan IIs and report to Congress next spring.

This month by Senate approved an amendment by Sen. David Pryor (D-Ark.) requiring installation of an early warning system that would alert people residing near the silos in the event of an accident.

Sen. Bob Dole (R-Kan.) has called, albeit quietly, for the Titans to be phased out.

Why are these aging missiles kept operational some seven years after they originally were scheduled for retirement?

Interviews with key White House and Pentagon officials indicate the primary reasons are political, rather than military.

Phase-out of the Titan IIs was announced publicly in early 1967. At that time the Air Force said it would halt the purchase of new missiles for test purposes.

"They became obsolete in 1971," a former officer in the Air Force missile program said recently. "They never were considered reliable," he said, and "never in a meaningful way fit into our force planning."

But the United States had hopes the big Titan II missile could be traded for the Soviet Union's heavy missiles in arms control negotiations, a White House aide recently said. To date, however, the Soviets have not bitten.

Nonetheless the Titan IIs were given new life, this time as a bargaining chip.They also gained symbolic meaning more recently in the domestic arena.

They are a psychological blanket," one Capitol Hill staff aide said. "They are the only big missile we have around to compare with what the Soviets have."

The primary U.S. land-based missiles are the 1,000 Minuteman IIs and IIIs. These missiles, much smaller than the liquid-fueled Titans, are considered safer because they use solid-fuel propellants.

They also are militarily more effective because of more accurate guidance systems and because, in the case of the 550 Minuteman IIIs, they carry three warheads instead of one.

All 1,000 Minuteman missiles, however, represent 820 megatons of nuclear destructive power. The 54 Titans account for 486 megatons.

Thus, should the Titans be dismantled, American overall megatonnage in ICBMs would drop by one-third, though total military effectiveness would not change.

At a time when critics of the administration and the stragegic arms limitation treaty (SALT II) are pointing out the large advantage the Soviets have in megatonnage, dropping the aging Titans could be politically disastrous.

Thus the Air Force has been told to keep the missiles operational and "on alert indefinitely," according to an Air Force spokesman.

This is turning out to be quite a challenge.

For one thing the cost is high. In fiscal 1979, $103 million was spent on the Titan IIs. That averages out to slightly less than $2 million for each missile.

In the strategic weapons business, where backup systems are considered important, the Air Force reluctantly admits there are only three spare Titan IIs around.

"We don't feel that's an unworkable number," the Air Force spokesman said recently. But he concedes that age is making it difficult to find new parts.

While other missiles in America's strategic arsenal undergo regular test firings each year, except for their nuclear components, there have been no such tests of the Titan II since 1976.

That last test was to see if a new guidance system put together from a Titan III space booster would work for the missile. The aged guidance system from the original Titan II had begun to fail, and there were no spare parts.

To take the place of the full-scale test launches, the Titan IIs are taken apart regularly for subsystem tests. Rocket engines, for example, are mounted on stands and ignited.

Data from those tests are compared with results from earlier launch tests to determine if there is any decline in the system's operation.

"We believe [the Titan II] alert status is as high now or higher than it's ever been," the Air Force spokesman said.

The tests, however, require draining of the Titan engine's highly toxic liquid fuel.

It has been during those activities that most of the accidents have occured.

Gilckman was the first to suggest the Air Force look into the safety of the Titan, and his call was picked up by Dole.

More recently Sen. Barry Goldwater (R-Ariz.) joined in the effort after leaks had occurred in Titan IIs based in his state.

A request from Goldwater, Dole and Pryor for a Senate Armed Services Committee inquiry into the missiles is being studied by Committee Chairman Sen. John Stennis (D-Miss.)

Pending these inquiries, the Air Force, according to a spokesman, remains firm that "there is no established phase out date for the Titan II."