In making a gradual transition from flamboyant Pentagon critic to the liberal who saved the MX missile, Rep. Les Aspin (D-Wis.) has managed to alienate colleagues on both the left and the right.

But the 46-year-old former economics professor retained enough respect for his factual approach to defense issues to win election yesterday as the new chairman of the House Armed Services Committee.

The decision by House Democrats to oust committee Chairman Melvin Price (D-Ill.), 80, in favor of a more junior member drew sharply contrasting reactions from different wings of the party.

"Nobody can question his credentials," Rep. Patricia Schroeder (D-Colo.) said of Aspin, who served as an Army economist under President Lyndon B. Johnson. "He's been a player at both ends, on the Hill and at the Defense Department. He's terribly knowledgeable about the budget . . . . I think he'll make the committee much more active, much more viable."

But a more senior Pentagon supporter on the committee, Rep. Samuel S. Stratton (D-N.Y.), was angry. "The House of Representatives has knocked out a guy who supported defense for 40 years and put in someone who represents the anti-defense Democrats," he said.

"This will make it much more difficult for us to get an arms control agreement with the Soviets," Stratton said. He said Aspin "wants to reduce our defense capability . . . . If the Democratic Party wants to continue to lose elections, this is the way to do it."

Aspin's years of churning out "Monday morning press releases criticizing the Pentagon," as Stratton put it, have left some conservatives distrustful of his commitment to national defense. But his role in the last Congress as the key broker in forging a compromise with the Reagan administration to continue the MX intercontinental nuclear missile disappointed many liberals.

"His friends and philosophical soul mates were really mad at him," said Rep. Thomas J. Downey (D-N.Y.). Still, Downey said, "his chairmanship was made possible by his support of the MX. He had been a loner . . . . He's learned to work with people over the years."

In recent days, Aspin has let it be known that he might be willing to move further toward the party's position against the MX now that arms control talks with the Soviet Union are back on track. Supporters have argued that the MX is an important bargaining chip in convincing the Soviets to negotiate on nuclear arms.

A consummate politician with a booming voice and a sheep dog named Junket, Aspin frequently courts reporters, lawmakers and defense specialists at small dinner parties in his Georgetown home. With a master's degree from Oxford University and a doctorate in economics from the Massachusetts Institute of Technology, he is at home in the intellectual arena.

Aspin learned the art of press-release politics as an aide to Sen. William Proxmire (D-Wis.) in the early 1960s.

He served briefly on the staff of President John F. Kennedy's Council of Economic Advisers and later became an economist in Robert McNamara's Defense Department. After a stint as a Marquette University professor, the Racine, Wis., native won election to the House in 1970 on an anti-Vietnam war platform.

Almost exactly 10 years ago, he helped topple the committee's autocratic chairman, F. Edward Hebert (D-La.), in favor of Price, setting a precedent that was to serve him well in yesterday's coup.

Never a doctrinaire liberal, Aspin bucked the political tide to support draft registration during the Carter years. But as recently as 1981, he was still castigating Defense Secretary Caspar W. Weinberger for spending too much on PX stores and for using military aircraft on his summertime flights to Bar Harbor, Maine.

Since becoming chairman of the subcommittee on military personnel and compensation in 1983, Aspin has moderated his style. After years of warning of a defense "readiness gap," he changed his mind and said the Pentagon may be spending too much on readiness. While backing the MX, he also declared that the administration has "very little credibility" on arms control.

Moreover, he has become more realistic about pruning the defense budget. "Congress doesn't cut big weapons systems, and it probably adds more waste than it cuts," Aspin told the National Journal.

Aspin aides say it is his image, not his opinions, that has changed. "When he was shut out, the only opportunity he had was to stand outside and throw rocks," one said. "But he is not and never has been anti-military. Once he was allowed to be an inside player, he decided to emphasize fashioning compromises on the floor."

The panel's ranking Republican, Rep. William L. Dickinson (Ala.), called Aspin "a very capable and smart guy" who will be more "adversarial" than Price on the defense budget. "He knows how to put out a release every week and criticize the Pentagon in a sensational manner . . . . But a lot of his criticisms have been based on fact," Dickinson said. "I don't think Les will try to kick over ash cans and make a lot of noise."

Added Downey: "He's still the chairman of a very conservative committee that is pro-defense. He won't have the votes to change its character."