The political cease-fire over the MX was broken yesterday when Chairman Les Aspin (D-Wis.) of the House Armed Services Committee threatened to delay the defense procurement bill if the Reagan administration does not accept a "permanent cap" of 50 deployed missiles.

Defense Secretary Caspar W. Weinberger aroused Aspin's concern when he told reporters Aug. 8 that he hoped to persuade Congress "that we need more than 50 MXs to accomplish the deterrence we have to have."

Aspin wrote Weinberger that he was disturbed by this apparent intention to push for more deployed missiles. "My view of the conference" in which the House and Senate attained a compromise authorizing $302.5 billion for the Pentagon in fiscal 1986, Aspin wrote, "is that the 50-missile cap is permanent," barring improvement in ways to base the missile or a change in the international situation.

"My personal view is that we need to get some better understandings before we bring the defense conference report to the House floor" for a vote, Aspin said.

The Senate passed a bill to suspend MX deployment at 50 missiles, while the House approved a permanent cap of 40. The compromise combined the House's permanent cap with the Senate's higher number. The missiles would be deployed in existing Minuteman missile silos.

In a related development yesterday, the Pentagon announced that a commission headed by John M. Deutch, provost of Massachusetts Institute of Technology, will assess the small intercontinental ballistic missile, nicknamed Midgetman, as directed by Congress.

Defense officials said the study, while focused on the Midgetman, could bring a recommendation to go back to the MX as the superior weapon. Such a recommendation would refuel the MX basing argument, which Aspin and other lawmakers thought had been put behind them in the legislation now pending.

The Midgetman report is scheduled to be submitted with Reagan's fiscal 1987 budget next year.

At the Pentagon, spokesman Fred S. Hoffman said that Weinberger has repeatedly argued that "50 MXs are not enough. We are hopeful that one way or another, Congress will eventually agree to the deployment of the full 100 in a suitable basing mode."

Hoffman said the Pentagon has no argument with the conference report language directing that no more than 50 MXs go into Minuteman silos. The Pentagon's focus is on the 51st missile and beyond, officials said. "We are currently studying alternative basing modes," Hoffman said, "so we see no reason for this issue to delay a prompt vote on the conference report."