Defense Secretary William S. Cohen's surprise decision to replace Army Gen. Wesley K. Clark as NATO's top commander ahead of schedule ran into some criticism on Capitol Hill yesterday. But there was no move to oppose it or to block Cohen's choice of Air Force Gen. Joseph Ralston for the job.

The selection of Ralston was swiftly endorsed by Sen. John W. Warner (R-Va.), chairman of the Armed Services Committee, which must vote on the nomination. Warner's early support reduces the chances that Ralston will run into the kind of political trouble he did three years ago, when the disclosure of an earlier adulterous affair caused him to withdraw from consideration to be chairman of the Joint Chiefs of Staff.

But some critics took issue with Cohen's decision to ease Clark out of the NATO post next April.

"My greatest fear is that the timing and the manner in which this announcement was handled could undermine General Clark's authority at a time when we are still trying to avoid catastrophes" in the Balkans, said Rep. Ellen Tauscher (D-Calif.), a member of the House Armed Services Committee. Instead of retiring Clark early, Tauscher said, the United States should award him a Congressional Gold Medal for leading NATO's air campaign against Yugoslavia.

Cohen and other senior administration officials defended the abrupt change of command, praising Clark but making clear that Ralston, now vice chairman of the Joint Chiefs, was their preferred choice to take over the NATO job.

Clark, who had expected to stay until July and possibly longer, learned Tuesday of the plan to have him step down next spring. He was informed of Cohen's decision while on an official visit to Lithuania, in a phone call from Army Gen. Henry H. Shelton, chairman of the Joint Chiefs.

Critics questioned why Cohen had not made the call himself and why Clark was caught off guard. The incident only served to highlight strains between the general and the defense secretary over the conduct of the war against Yugoslavia.

Sen. John McCain (R-Ariz.), an Armed Services Committee member and presidential candidate, expressed concern that if Clark's early departure "is in any way punishment for giving the commander-in-chief his best advice and counsel" during the air war, then it "will influence other senior officers to place political considerations before military necessities and deprive future presidents of the counsel they need to best protect our security."

White House and Pentagon officials vigorously rebutted suggestions that Clark was being victimized because of his advocacy during the war of more aggressive airstrikes, the use of Apache attack helicopters and the positioning of ground troops for a land invasion. They also expressed confidence that Ralston will win confirmation.

A highly decorated jet fighter pilot who fought in the Vietnam War, Ralston, 55, had been Cohen's first choice for chairman of the Joint Chiefs in 1997. But word of an affair he had in the 1980s while separated from his wife coincided with a highly charged national debate over whether the military was applying a double standard in sexual misconduct cases against senior officers and lower-ranking service members. The Air Force sparked the debate by compelling its first female bomber pilot, 1st Lt. Kelly Flinn, to choose a less-than-honorable discharge to avoid court-martial for adultery, lying and disobedience.

Rather than risk a bruising political fight, Ralston withdrew. He won easy confirmation the following year when put forward for a second two-year term as vice chairman.