Washington — The political fallout from this week's elections casts a darkening shadow over the Reagan administration's mighty military machine.
According to inside congressional participants from both political parties as well as outside expert observers, it now will be more difficult for the advocates of strengthened national security to prevail in the budgetary debate, especially in the House of Representatives.
''There's no question that it's going to be a more liberal House,'' says Rep. G. William Whitehurst (R) of Virginia, a member of the House Armed Services Committee. ''The (defense) budget is going to have tough sledding.''
Ticking off a list of Pentagon friends who will not be returning to Capitol Hill in January, another Republican insider says, ''That's a lot of votes.''
''More importantly, the people coming in have been talking about cutting defense spending and protecting social security,'' says this informant. ''We can expect them to take a rather stern view of the Defense Department.''
Common Cause president Fred Wertheimer observes that military spending in fact ''was a more intense issue'' than the nuclear freeze in this year's campaign.''
''Clearly, in my view, one of the messages coming out of this election - in the context of the economic question - directly relates to getting a handle on military spending,'' says Mr. Wertheimer. ''The numbers in the House do not bode well for the administration.''
This may be a direct reflection of the changing public perception about the need for a defense buildup. According to polls taken by the National Opinion Research Center in Chicago, the strong defense consensus that accompanied Ronald Reagan's election two years ago has waned significantly. Study director Tom Smith says many Americans now believe the country's military might has been sufficiently restored.
''People are convinced that the redress in the imbalance they saw earlier has been accomplished already,'' said Mr. Smith.
Even before this election there was growing concern that the administration's unprecedented peacetime military buildup would add to mounting federal deficits and thereby hurt the economy.
Employment Research Associates in Lansing, Mich., reported just a week before the election that ''every $1 billion transferred from purchases by the taxpayer to purchases by the Pentagon caused a net loss of 18,000 jobs in industry and commerce.''
Former Defense Secretary James R. Schlesinger recently told defense contractors that record deficits resulting from, in his view, unwise tax cuts would lead to defense budgets below even what the Carter administration had planned.
The conservative Heritage Foundation has analyzed the $12 billion in cuts already made in the Pentagon's 1983 budget authority and found that the result is a sum ($214 billion) only $4 billion more than Jimmy Carter had proposed.
While Congress has yet to strip the military of any planned ''big ticket'' weapons systems, it is likely to continue nibbling away at the overall sum. And such items as the two proposed nuclear aircraft carriers, which along with their aircraft and support ships would cost many billions of dollars over the next several years, are far from home free.
Some observers note that there is a way for the White House to reduce defense spending without cutting weapons or programs: Since inflation has dropped considerably in recent months, the Pentagon could simply reduce its built-in budget hedge against inflation and claim spending cuts totaling billions over the next few years. Some administration budgeteers are urging this, but the President may not want to send any signal of a defense pullback to the Soviet Union.
There are two early signals of congressional intent to look for.
House Speaker Thomas P. O'Neill Jr. (D) of Massachusetts may decide to withhold the Pentagon's budget for the current 1983 fiscal year from full House debate and voting until after the lame ducks have been formally retired in January. Keeping the Defense Department afloat on continuing resolutions, says a Republican source on Capitol Hill, ''would be very bad news for the Pentagon.''
Another early warning for the Pentagon could be action on the administration's strategic nuclear buildup. The House very narrowly defeated a nuclear freeze resolution earlier this year, but a New York Times/CBS poll shows that a majority of new House members favor a freeze.
Voter approval of a nuclear freeze in eight of the nine states where it was on the ballot also sends a clear signal to lawmakers. ''I would guess that the nuclear freeze vote seems likely to strengthen the antidefense forces in both houses,'' says national security analyst Neil Singer of the Congressional Budget Office.
Also coming soon is the White House proposal to base the new MX strategic nuclear missile. There is growing concern among defense boosters as well as Pentagon critics that the likely basing mode - ''dense pack'' - still may leave the $25 billion missile system vulnerable.
How Congress reacts to the cost, strategic wisdom, and arms control ramifications of the MX could send clear signals to the military establishment regarding its future budgetary desires.