House Armed Services Committee Chairman Les Aspin (D-Wis.), who just three months ago was accused of betraying his party by backing President Reagan on the MX missile, is now leading an effort to fashion a Democratic alternative to President Reagan on defense.

As unveiled to the House Democratic Caucus in closed session last week, the alternative would significantly alter priorities in the 1986 defense bill approved by Aspin's committee. The purpose of the alternative is to show that the Democrats have a defense program of their own, which to some extent shifts priorities away from nuclear forces and toward conventional ones.

"It's a reordering," Aspin said.

The alternative has also helped Aspin return to the good graces of many in the Democratic caucus who felt betrayed when he voted with Reagan and against a majority of the caucus to back further production of the MX this year.

But in taking the unusual -- possibly unprecedented -- step of working on amendments to change substantially his own committee's bill, Aspin may be creating problems for himself as chairman.

"The committee is in turmoil," said one of the members, Rep. Samuel S. Stratton (D-N.Y.). "As chairman, he ought to represent the committee and not come out for a substantial change in the committee's bill."

Aspin said he did not try to get his committee to put the proposal in the defense bill because he was waiting for the Senate to act and because most of the issues addressed in the alternative would be decided on the House floor anyway.

But others said Aspin probably would not have been able to push the package through the Armed Services Committee, which is dominated by a coalition of Republicans and conservative Democrats.

"He has two different constituencies that are competing," one Democrat said. "The full committee, which is basically conservative, and the Democratic caucus constituency, which is much more moderate. That's his problem now."

Aspin is to present the committee bill when it comes to the floor; the Democratic package will be offered as a series of amendments by a group of Democrats, including several of the younger, more activist lawmakers, and at least one moderate Republican.

The alternative package would permanently cap deployment of the multiwarhead MX missile at the number of missiles already approved -- 40 -- and would provide no MX funds next year. It would prohibit further tests next year of antisatellite weapons in space.

It would also cut almost in half -- from $3.7 billion to around $1.9 billion or less -- Reagan's request for research on a space-based strategic defense system, known as "Star Wars," and place a lid on programs that might violate the 1972 Anti-Ballistic Missile Treaty.

At the same time, it would increase research funds for the small, mobile single-warhead Midgetman missile that many lawmakers believe is more survivable than the stationary MX. The package would also take the money saved from the cuts in MX and Star Wars and devote most of it to improvements in conventional forces, for such things as ammunition, so-called "smart" weapons and air-to-air missiles.