THIS IS D-DAY on Zimbabwe Rhodesia, the day the House will decide whether to follow the Senate in ordering the president to lift sanctions. But, interestingly enough, the atmosphere is distinctly different from what it was when the Senate acted 10 days ago. In the interim, the House Foreign Affairs Committee, by an astonishing vote of 33 to 0, recommended in effect to leave the final sanctions decision to the president. The committee did this by agreeing that sanctions should be lifted by Oct. 15 unless the president determines that it would not be in the national interest to do so.

What happended to bring about this change, which, if sustained, will probably produce a stalemate satisfactory to the administration? Two things. First, in its lobbying of the House the administration began adding to its warnings of impending diplomatic disaster a new emphasis - pleasing to congressmen - on the positive aspects of the Muzorewa regime in Salisbury. By this tactic, officials eased the argument away from the divisive question of the democratic merits of the recent Rhodesian elections and toward the more conciliating question of how to preserve and strengthen the gains made. If the administration had subdued its seemingly dogmatic fixation on the guerrillas months ago, it might never have gotten in its present fix at all.

Then, appreciation has grown in the House of the changing international context in which the sanctions question now must be addressed. With the new government in London, it is clear that the British are virtually certain to lift sanctions against their former colony by the year's end and that the United States will automatically follow suit. So in a real sense the sanctions battle is over: They will end in just a few months - regardless. That makes the relevant question how best to use the intervening time to bring peace, which is not only a desirable goal but a condition necessary to consolidate democracy.

Whether the diplomacy contemplated by London and Washington can help bring progress in the next few months is problematical. The likelihood of success is very small. But the question of whether the Congress, by lifting sanctions, can advance peace is simpler to answer: It can't. Like it or not, only the diplomats have the scalpel necessary to attempt the operation. The Congress' instrument is blunt.