Defense Secretary Caspar W. Weinberger said yesterday that the Pentagon will recommend new basing modes for the MX missile in an effort to get around the congressional cap limiting its deployment in existing Minuteman silos to 50.

Reviving a controversial issue that many members of Congress consider dead after 11 years of debate, Weinberger said the 50-missile limit, half of the Reagan administration's request, leaves the U.S. land-based strategic force too weak to deter a Soviet nuclear attack.

"We were told the MX program was dead about 10 times over the 11-year period, and we now have 50," Weinberger said of his hope that more can be obtained.

His comments, at a luncheon meeting with reporters, came two weeks after a House-Senate conference committee agreed to place a statutory limit of 50 on the number of the missiles deployed in Minuteman silos. Conferees left open the possibility of more missiles, however, if the administration found a more survivable basing method.

Weinberger said that if Congress believes Minuteman silos are too vulnerable, "we will tell them some other modes" of basing the MX. He cited an expensive superhardening process being developed.

"I'm hopeful that one way or another . . . Congress will accept the fact that we need more than 50 MXs to accomplish the deterrence we have to have," he said.

Discussing other weapons systems in the one-hour session, Weinberger said:

*He opposes any move to stop research on President Reagan's Strategic Defense Initiative, the "Star Wars" missile defense system, in exchange for "any kind of chimerical pledge" by the Soviet Union to reduce its offensive nuclear weapons.

*The Pentagon is "vigorously and effectively" developing the small, mobile Midgetman missile favored by many members of Congress, although "it is difficult to deal with" congressionally mandated design specifications, including weight limits.

The MX cap was part of the conference committee's $302.5 defense authorization bill, scheduled for a vote in both chambers next month.

The House had sought to freeze the number of missiles at 40, but Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee and chief House negotiator in the conference, said later that the most important result was that "the MX is dead."

Other members of Congress said so many other MX basing strategies have been rejected in the past that the administration would give up the battle.

But Weinberger breathed new life into the MX program yesterday, indicating the administration will propose ways to base the missiles in less vulnerable sites.

He defended deployment in Minuteman silos as the way to "regain most quickly the deterrent capability that you need," but he added that "if Congress doesn't want any more than 50 in there, then obviously that's Congress's will. But we will need more than 50, so we will be suggesting some other ways to achieve that capability.

"I think Congress understands its not enough, because they have not said, 'You can't have anymore.' What they've said is, 'You have to give us a basing mode that we find more acceptable.' "