The Central Intelligence Agency is apparently confident that it can bamboozle senators, but it doesn't want to take chances with Senate staffers who might be expert enough to challenge the agency with embarrassing questions.

This, at least, is the conclusion we draw from the fact that Senate aides -- even those with top security clearance -- have been systematically excluded from CIA briefings on such sensitive matters as nuclear nonproliferation. The policy has been in effect for several months, and State Department officials who have gone along with it told our associate Lucette Lagnado that they are "only following orders" -- presumably those of CIA Director William J. Casey.

The result is that senators get their secret briefings from CIA and State Department officials without the presence of staff aides whose special knowledge might give them the chance to bring up the finer points of the issues involved.

This disturbing attempt to restrict access to important information is illustrated by the Senate Foreign Relations Committee's briefings on Pakistan and its development of the "Islamic bomb." For years it has been suspected that Pakistan has been trying to build a nuclear bomb. If Pakistan is known to have a bomb or to be working on one, U.S. military aid must by law be cut off.

In the face of Pakistani denials, the issue hinges on technical niceties: What kind of enriched uranium is Pakistan producing at its nuclear plant in Kahuta? Is it only 5 percent, as the United States has demanded, or 30 percent, as intelligence reports have suggested? Ninety percent enriched uranium is necessary to produce nuclear weapons.

Although Pakistan has apparently shut down its plant that reprocessed plutonium, its uranium-enrichment factory is churning away, according to our sources. And because India has reportedly been expanding its nuclear weapons capability since it first tested a nuclear device in 1974, it is a matter of serious concern to the United States that Pakistan seems determined to join the "nuclear club."

It happens that the United States' six-year, $3.2 billion aid program to Pakistan is about to run out. The Reagan administration wants to renew it for another six years, with an increase to $4 billion. But Congress mandated that aid would be cut off if there was solid evidence that Pakistan had built a nuclear weapon.

Last fall, we're told, the White House prepared a legal brief that insisted -- if somewhat gingerly -- that Pakistan has not produced a nuclear weapon. But our sources also say that there was serious dissent within the administration over the reliability of this brief.

This is what makes the matter of congressional briefings by the CIA so critical. Senators have too many other things on their minds to be able to quiz their CIA briefers properly on the arcane issue of Pakistan's nuclear capability. But staff aides who might know the right questions to ask have been locked out of the briefing room.

Thus, the Senate will vote in the dark on renewal of aid to Pakistan, and another nuclear power could be born with America as an unwitting midwife.