If Sen. Sam Nunn were a vengeful man, or as politically infantile as some Reagan administration officials he's had to deal with, he would be laughing his head off.

But of course he isn't. Nunn is a political grown-up; and neither he nor others who saw it coming can take pleasure in the jam the administration is in over the Senate approval of treaties.

Forget for the moment the merits or demerits of the recently signed INF Treaty, which the Senate must consider in the new year. The administration would obviously like the senators to address the new pact speedily, and without attaching troublesome reservations.

To avert the latter possibility, the administration must now sit down to a preliminary course of crow -- of its own cooking. This unwelcome first course is the direct and predictable result of a notable Reagan administration folly: the attempt to rewrite the 1972 ABM Treaty so as to facilitate "Star Wars" testing.

Two years ago, armed with a political brief signed by the State Department's chief counsel, the administration announced that it had a new interpretation of the ABM Treaty -- one that seemed strange and novel even to those who had negotiated it. Under this "broad" reading of the treaty, the testing of space-based antiballistic missile systems would be permitted, notwithstanding 15 years of contrary interpretation.

The administration claimed that the "negotiating record" sustained the new, permissive reading. Indeed, from the administration's point of view the "broad" reading was a wonderful convenience.

From the Senate's point of view, it raised all sorts of disturbing questions that went well beyond the provisions of a single arms-control treaty. The main question was this: Whom do you believe, and when, about the meaning of a treaty? When the Senate debated ABM in 1972, its Nixon administration sponsors assured the Senate that the "narrow" interpretation was right. As Nunn demonstrated last spring in a devastating series of Senate speeches, the floor debates of the time left no doubt.

Now, suddenly, a successor administration was claiming that the Senate had been misinformed, that the "negotiating record," which senators hadn't bothered to examine in detail, undercut the representations they had believed. It was as if the fellow who sold you a lemon suddenly claimed 15 years later that whatever he'd called it back then, or whatever it tasted like, it really was a grapefruit.

It is easy to see -- as Reagan officials were quickly and clearly warned -- why such double-dealing would not work and would inevitably complicate future dealings between the White House and the Senate. There are ways of distinguishing on the spot between a lemon and a grapefruit. But that is not the case with treaty language, some of which is inescapably ambiguous.

Was the Senate making a mountain of a molehill? That's what the administration's political stooges claimed when Nunn challenged the new interpretation. They dismissed his worries as politically inspired; it was all, they whispered, grist for his presidential campaign.

Today, as the State Department sheepishly petitions the Senate for a quick and painless consideration of the new INF Treaty, the administration is reaping the consequences of crafty lawyering, arrogance and bad judgment.

Why should the Senate care what the executive branch tells it about a treaty? The question answers itself. Senators do not negotiate treaties. But they must approve them -- certify, in effect, that they are in the nation's best interest. In discharging that duty, they can take one of two approaches. They can assume that the treaty negotiators are making good-faith representations. Or they can laboriously comb the "negotiating record" for themselves, seeking traps and double meanings.

The first procedure, which is customary, is obviously better for all concerned. The two political branches should be able to assume that they are dealing with one another candidly, and as adults.

Bare months ago, the Senate was being told that the partially secret "negotiating record" is the key index to the meaning of a treaty. Could that be the case with INF, then?

The administration, having drastically changed its tune, is now entreating Nunn and others not to believe that it is guided today by what it was saying only yesterday in the misguided effort to elasticize the ABM Treaty.

Will Nunn and the Senate believe the old tune or the new?