Members of a presidential commission acknowledged before Congress yesterday that political and environmental pressures, rather than purely military factors, figured most prominently in their unanimous recommendation that 100 new MX missiles be placed in existing Minuteman missile silos.

They also acknowledged in response to questions that, had they been able to put aside these nonmilitary considerations and start over on the problem of how to base the MX, they would have favored an elaborate "shell game" of the sort advocated in the Carter administration in which about 200 missiles would have been hidden among thousands of shelters to improve their chances of surviving an attack.

But commission chairman Brent Scowcroft, former Carter secretary of defense Harold Brown, and former dean of the Massachusetts Institute of Technology John M. Deutch all told the Senate Armed Services Committee that it was too late to start over now.

There is an urgent need, they said, to approve the panel's recommendations, which are expected to be endorsed by President Reagan today.

Their appearance kicked off what are expected to be extensive hearings in the Senate and the House on the controversial report released by the 11-member MX panel earlier this month.

Aside from recommending that the big multiple-warhead MX be fielded quickly in existing silos to begin to offset Soviet weapons with similar capabilities, the panel also urged development of a much smaller single-warhead missile for the 1990s that might be more difficult for the Russians to find and hit and thus perhaps reduce the temptation of either side to strike first in a crisis.

Although the commission's work was praised by such influential senators as Barry Goldwater (R-Ariz.) and Sam Nunn (D-Ga.), its recommendations to put MXs into theoretically vulnerable existing silos ran into considerable skepticism from a number of Democrats and two leading Republicans, who questioned the role of politics in what was ostensibly a military decision.

Sens. J. James Exon (D-Neb.) and Carl Levin (D-Mich.) emphasized that for years all the experts and Congress had said that the Minuteman silos were vulnerable to attack and that Reagan had campaigned hard on this point in 1980. But now the panel wants to put the missiles in such silos anyway. Exon called the panel's work "more of a political report and a political consensus rather than a military one concerning the security of the United States."

"Environmental and political considerations really have dictated the commission's current dilemma," said Sen. William S. Cohen (R-Maine). Alluding to the objections of western states to previous MX basing schemes and Reagan's rejection of Carter's plan, Cohen said the need was "to find a basing mode that would suit this president and satisfy the western objections. Isn't that essentially where we are?" he asked Scowcroft.

"Yes, it is," the chairman acknowledged.

Sen. John W. Warner (R-Va.) repeatedly came back to the question of "what is the best system militarily for the nation?"

"Politics be damned," he said, adding that "I think we have a responsibility to go back and reexamine" the multiple-shelter plan proposed by Carter, which Scowcroft and Brown had supported in the past.

The exchange with Warner captured one of the basic challenges the administration now faces on Capitol Hill. Officials who in the past argued strongly against use of Minuteman silos on grounds that they were vulnerable to attack are now trying to convince Congress that vulnerability is not that serious a problem because they believe that a Minuteman silo plan is the only one possible politically and that it is crucial that the MX missile be deployed any way it can.

"There may, looking back in history, have been better ways to do this," said Brown. "However, I think we are now facing a situation where we have to proceed from where we are now. I think there is no way to go back."

Warner also put his finger on a second key question certain to figure prominently in a congressional decision: why not skip MX and go directly to the small missile being considered as a successor to MX?

Brown and Scowcroft said that neither the Russians nor anyone else would believe that the United States would actually wind up with such a missile if it failed to field the MX first to show its determination to bolster its forces. They also said the small missile was many years away and there were no guarantees that it would not run into political trouble as did the MX.