THINK HARD, NOW. What is the most efficient and certain way the Reagan administration might ensure the collapse of its No. 1 foreign policy project, securing Senate approval for ratification of the INF Treaty? This problem may already have been solved. The administration, by prolonging its dispute with Sens. Nunn and Byrd, may be doing everything necessary to derail the treaty and consummate a political, diplomatic and strategic disaster.

The trouble arises from the course President Reagan has chosen on his Strategic Defense Initiative. When he wanted to open the door to ambitious testing of this space-based missile defense system, he found that the Anti-Ballistic Missile Treaty of 1972 barred the way. The lawyers, however, came up with something: the notion that although the way was barred by what the executive branch had told the Senate the treaty meant at the time, the way was actually opened by what the treaty's secret negotiating record revealed.

Right at that point, a flag should have gone up over at the State Department. Somebody should have said: Wait a minute. What about the next time? Suppose the Senate asks how it can believe what we're saying about the next treaty? Instead, State Department legal adviser Abraham Sofaer said . . . Well, what he said is what has Sam Nunn and Robert Byrd insisting that if the administration is not prepared to vouch for its own testimony before the Senate, then the Senate is going to have to examine the (30-volume) negotiating record ''exhaustively'' and meanwhile put the INF Treaty on hold. Secretary of State George Shultz appeared ready at one point to accommodate to the Senate position but then, it's reported, backed off.

Hardball? Yes. A small point of Senate privilege? No, a large point: the Senate cannot possibly be asked to approve a treaty when the administration reserves a right to say later that it means something other than what the executive branch asserted at the time. A point on which the public (and perhaps other senators) will not support the insisting senators? An administration with the ambitious foreign policy agenda of this one would have to be very careless to make a full-scale test. A point important in order to preserve a broad SDI testing option? If that's so, it's better to suspend the INF debate right now and sort out the cluttered SDI issue before the president goes any farther down the diplomatic path.