Like a shadow government - but one with considerable clout - the conservative Heritage Foundation is at work throughout the Reagan administration. Its fingerprints can clearly be seen on the administration's 1986 budget, now emerging from White House deliberations. And its access in recent days to top government officials, including Cabinet secretaries, has been unprecedented for a private organization.
Four years ago, the group's ''Mandate for Leadership'' - a doorstopper of 1, 000-plus pages outlining ''policy management in a conservative administration'' - was gratefully accepted by a newly elected Ronald Reagan as ''most useful.'' It was a blueprint for much of the ''Reagan revolution'' that united Republicans and conservative Democrats to push the new President's budget and policy plans through Congress. A year later, foundation officials estimated that more than 60 percent of their 2,000 proposals had been completed or begun by the administration.
Today, what Mr. Reagan has called ''that feisty new kid on the conservative block'' is publishing its follow-up report. ''Mandate for Leadership II: Continuing the Conservative Revolution'' is also long (566 pages). And, with more than 150 analysts and public policy experts contributing, it is meaty.
The conservative organization lauds the administration in some areas, but is critical in others. Some of its new recommendations are part of White House plans for Reagan's second term. Others were rejected as too unrealistic or politically sensitive for the administration to accept.
In its second term, states the Heritage report, the administration faces ''a very different challenge'' than it did when it first took office, since it now is ''burdened by the day-to-day pressures of governing.''
''After a seeming invincibility in the first year of the term,'' observes the report, ''Reagan officials gradually appeared to lose their edge. . . . The bureaucracy began to dig in, while the opposition in Congress organized more effectively.''
Stuart Butler, director of domestic policy studies at the foundation, writes in the report: ''The administration's solid victory in the polls this November masks a disturbing loss of momentum and sense of direction within the government.''
On defense and foreign policy, the conservative group is more pleased with the Reagan record.
With its military buildup, decisive move in Grenada, tough stance with leftist governments, and encouragement of democratic evolution in ''friendly authoritarian'' states, the administration ''has had a profound impact upon international politics,'' the report says.
On the other hand, W. Bruce Weinrod, director of foreign policy and defense studies at Heritage, writes in the report that ''the administration also has not conveyed a complete sense of the goals of US foreign and defense policy.''
The administration is faulted in the Heritage report for ''confusion'' in its attempts to base the MX missile, ''failure to coordinate adequately strategic trade policy,'' not providing enough assistance to Afghan freedom fighters, and ''committing US forces to Lebanon without sufficient evaluation of its implications.''
The report concludes that, ''on the administrative level, the first Reagan administration was 'business as usual.'
''As in most presidential administrations, many political executives in the first Reagan administration were captured, worked for narrow interest-group goals, and, at times, fought among themselves. The lack of action on the part of the political executives allowed the bureaucracy to maintain its power and continue to grind out the expansion of the federal government.''
The report asserts: ''The second Reagan administration must be different. The political executives in the second Reagan administration have an opportunity to control . . . the federal bureaucracy and to produce fundamental policy change. If, instead of a tiny minority, the majority of political executives are successful, they will contribute to the success of the Reagan presidency, which will be noted in history for reversing the course of the expansionist federal government.''
Drafts of the Heritage report were distributed to top officials by presidential counselor Edwin Meese, who called it ''one of the most meaningful and best things that President Reagan and those associated with him will have to guide them in the next few years.'' White House spokesmen say the foundation's recommendations have been ''useful'' in formulating the new budget package.
A better indicator of the Heritage influence may be its access to senior policymakers. In recent days, senior officials from six departments and agencies have been briefed by Heritage experts. There have been personal briefings for Secretary of State George Shultz and Agriculture Secretary John Block, and meetings with three more agencies are scheduled for next week.
Invitation acceptances to briefings for senior political appointees and key congressional staff members meant standing-room-only crowds at foundation headquarters Thursday.
Before the report's official release today, there were more than 300 calls from government agencies for copies of the draft. ''By and large, the reaction has been very positive,'' said Gordon Jones, foundation vice-president for academic and government relations.