The Reagan administration has been quietly and carefully preparing for a major battle over Senate approval of the U.S.-Soviet treaty eliminating Intermediate-Range Nuclear Forces (INF), despite outward signs that it expects only a brief skirmish.

Every weekday morning since Jan. 1, a group of 20 State Department officials has met secretly to plot detailed strategy for fending off any changes the Senate might propose to the treaty, on subjects from the balance of non-nuclear forces in Europe and a future pact on long-range nuclear arms to the Soviet incursion in Afghanistan and its record of treaty violations.

This group in turn has reported regularly to two more senior groups led by White House chief of staff Howard H. Baker Jr. and national security adviser Colin L. Powell and by Lt. Gen. Powell's staff.

These groups have canvassed and briefed dozens of Senate staffers, prepared testimony for administration witnesses and compiled a record of treaty negotiations to be sent to the Senate this week. Nine supporting documents will be included in the official treaty package to be given by a White House courier to the Senate clerk Monday morning, including a 120-page line-by-line treaty analysis drafted in recent weeks by administration lawyers and arms control negotiators.

Much of the administration's caution stems from the novel role it will be forced to play in the Senate hearings beginning Monday. A conservative government that for years emphasized growing defense budgets and anticommunist rhetoric will now implore a Democratic-run legislature to support an arms-reduction pact with the Soviet Union. Some officials worry that the circumstances are ripe at least for political embarrassment.

Senate conservatives and liberals alike have pledged, for example, to demand an explanation for President Reagan's approval of the INF Treaty despite his continuing belief that the Soviets have violated past agreements and his assertion last spring that Soviet compliance with previous arms treaties was "an essential prerequisite" for future pacts.

Several senators also have vowed to question whether elimination of 859 medium- and shorter-range U.S nuclear missiles would endanger Western security, given past administration assertions that the Soviets maintain a vast advantage in non-nuclear arms they could use to attack U.S. allies in Europe.

Officials predict that some senators will use the hearings as a forum for debating past administration arms policies and the direction of U.S.-Soviet relations.

Sen. Jesse Helms (N.C.), ranking Republican on the Senate Foreign Relations Committee, has said that "we must not get lost in the technicalities of verification, compliance and missile technology," but should instead "examine all aspects of the U.S.-Soviet relationship" during the hearings.

"I think we'll have all kinds of nasty questions," senior State Department arms control adviser Paul H. Nitze said last week. But "I'd be surprised if {the Senate} cooked up" an INF Treaty amendment that effectively overturned six years of negotiations that were closely observed by a bipartisan Senate group, he added.

A variety of attempts to link the INF Treaty to other issues, such as the balance of non-nuclear arms, could be problematic for the administration if they come from treaty supporters such as Sen. Sam Nunn (D-Ga.), officials said last week.

The administration's principal recourse, in response to such demands, will be to emphasize how modest the treaty's objectives are, and how awkward it would be to renege on its commitments to the Soviets about the scope of the treaty.

Officials said Secretary of State George P. Shultz and others will emphasize that the treaty eliminates more Soviet missiles than U.S. missiles, and that it does not bar improvements to remaining U.S. nuclear and conventional forces in Europe.

U.S. officials also expect some complaints about the treaty's text, including provisions circumscribing on-site inspections of U.S. and Soviet territory, and permitting reuse of the nuclear components of eliminated missiles in new weapons.

The administration will also provide the Senate with nearly 200 diagrams and photographs of U.S. and Soviet nuclear missile facilities, which were exchanged in the final stages of the negotiations but kept secret until now. Several officials said they expected this information to clear up some complaints about the pact's provisions to verify Soviet compliance, but another official suggested that it would simply create new opportunities for "mischief-making" by treaty opponents.

The administration has decided to provide the Senate with the complete, classified record of the INF negotiations this week, officials said. Late last year, key senators demanded to see the negotiating record because the administration had argued, in discussions of the 1972 Antiballistic Missile (ABM) Treaty last year, that statements made by negotiators determined the true meaning of the treaty.

The senators, including Nunn, argued that under the administration's rationale they would not know what they were approving without first inspecting the INF negotiating record.

Until the ABM debate, secret treaty negotiating records were ordinarily retained by the executive branch. But under a tentative new agreement, a record of every conversation between U.S. and Soviet negotiators in Geneva and between senior U.S. officials in Moscow and Washington on the INF Treaty will be made available to senators and staff in the Capitol Hill office formerly occupied by the Iran-contra investigating team.

Several administration officials said last week they are concerned that portions of the INF record might be used by treaty opponents to mislead the Senate or the public about the course of the negotiations or their final outcome.

Maynard W. Glitman, former chief INF negotiator, told reporters when the INF Treaty was signed, for example, that "if you go through a negotiating record you're going to get a lot of ins and outs and you're going to see a lot of posturing that both sides did in order to make certain points, and I think it'd be very easy to misread the actual flow of what was happening."

Another negotiator said, "How would you like it if someone wrote down everything you said and did for six years, and then gave it to some strangers?" The officials denied that anything in the record was acutely embarrassing, but said it might appear to treaty opponents as if the United States had made too many compromises with the Soviets -- an allegation that the administration vigorously denies.

Officials said the administration's objective is to assist treaty supporters in wrapping up the hearings by the end of next month, allowing ample time for debate in committee and a final vote on the Senate floor before the start of the Republican and Democratic presidential conventions this summer.

Once the Senate renders its "advice" on the treaty, the administration will reclaim the original document that Reagan and Soviet leader Mikhail Gorbachev signed. Until then, it will remain in the office of the Senate clerk, as a hostage to the separation of powers.