CIA director William H. Webster yesterday stepped into the middle of the congressional debate over potential war with Iraq by writing a letter saying that economic sanctions alone would not force an Iraqi withdrawal from Kuwait for at least a year.

Webster's highly pessimistic appraisal of the utility of economic sanctions was promptly disclosed by House Armed Services Committee Chairman Les Aspin (D-Wis.), making it a point of bitter contention between legislators who favor prompt military action if necessary and those who prefer to give sanctions more time to work against Iraqi President Saddam Hussein.

Senate intelligence committee Chairman David L. Boren (D-Okla.), who supports waiting, said Webster's letter to Aspin suggested that the CIA director was trying not to "cast doubt on decisions the president has reached" about the insufficiency of sanctions alone to persuade Saddam to withdraw his forces from Kuwait. He noted that "factual data presented by the CIA" to his committee, some of it in classified form, allowed "very different conclusions" to be drawn about the effectiveness of economic sanctions.

Legislators on each side of the debate yesterday offered what they said were the best intelligence estimates to bolster their case, illustrating that there remains considerable ambiguity about the role of sanctions in changing Saddam's mind.

Controversy over the sanctions issue has been building for several months. One group -- which includes Boren, Senate Armed Services Committee Chairman Sam Nunn (D-Ga.), House Foreign Affairs subcommittee Chairman Lee Hamilton (D-Ind.) and others -- asserts that the economic embargo will either avert war, by pressuring Saddam to withdraw, or will at least degrade the effectiveness of Iraqi forces in eventual combat.

The other group -- which includes Aspin, House Foreign Affairs Committee Chairman Dante B. Fascell (D-Fla.), Sen. Minority Leader Robert J. Dole (R-Kan.) and others -- has emphasized that any delay in military combat could benefit Saddam more than his opponents.

No one disputes that the sanctions are seriously damaging the Iraqi economy. The debate centers on whether the damage will be enough to reverse Saddam's determination to retain control of Kuwait or diminish the loss of U.S. life in conflict with weakened Iraqi forces.

Webster said in testimony before Aspin's panel Dec. 5 that while economic sanctions "have dealt a serious blow" to Iraq, "there is no assurance or guarantee" the resulting hardships will force a reversal of Saddam's takeover of Kuwait. He also said the embargo will degrade the readiness of Iraqi military aircraft within three to six months and probably begin to harm Iraqi armored ground forces in nine months.

In yesterday's letter, Webster went beyond this statement to offer a more detailed assessment that "the ability of Iraqi ground forces to defend Kuwait is unlikely to be substantially eroded over the next six to 12 months even if effective sanctions can be maintained."

Webster said that while some armored vehicles would be idled and crews' skills would get rusty during this period, the resulting "marginal decline of combat power" probably would be offset by improvements that Iraqi forces could make in their defensive fortifications with additional time. He also noted that the expected degradation of Iraq's air force would have "only a marginal impact on Saddam's ability to hold Kuwait" because his airplanes will not play a "major role in any battle."

Altering his previous public testimony that "there is no assurance or guarantee" sanctions will force Saddam's hand, Webster said in the letter that "economic hardship alone is unlikely to compel Saddam to retreat from Kuwait" even if sanctions remain in place for six to 12 months.

Several key Democratic aides said in interviews that in offering these new judgments, Webster selectively presented negative information while omitting information shared by CIA analysts with Congress as recently as Wednesday on how well the sanctions were working.

Senate Majority Leader George J. Mitchell (D-Maine) quoted the CIA as predicting, for example, that continuing sanctions will cause "the virtual depletion of Iraq's foreign exchange reserves by spring" and "the shutdown of nearly all energy-related and military industries by the summer" as well as "a severe reduction in basic commodities."

"This {Webster letter} is more a policy letter than an intelligence assessment," said an aide to a key opponent of using force now against Iraq. Another Democratic aide said: "I'm a little disturbed that we're getting selective declassification" of intelligence data in the midst of the debate.

CIA spokesman Mark Mansfield said Webster's letter was written in response to Aspin's specific questions, and "a careful reading would indicate it is consistent with his {past} testimony" and "does not represent a change in the intelligence community's assessment."