The bureaucratic battle to shape President Carter's impending decisions on the future of the U.S. strategic nuclear force has entered its last and most public phase.

That battle, which cuts through the entire national security bureaucracy in the Pentagon, State Department and White House, centers on such major issues for presidential determination as:

The target killing power and numbers of the next generation of U.S. intercontinental ballistic missiles (ICBMs). These are the land-based MX and the submarine-launched Trident II.

The targeting doctrine determining the priorities with which Soviet cities, as opposed to hardened missile launching sites, will be selected for destruction.

The public rationale that explains how the Carter administration's strategic force would deter the Soviet Union from starting a nuclear war.

Riding on the president's choices will be the next round in the strategic arms race with the Soviet Union and, quite possibly, Senate approval of any agreed-upon strategic arms limitation treaty (SALT).

Sources inside and outside the administration said last week that the president has been hesitant in making the required decisions -- thus pushing the bureaucratic fight out in the open.

The first strike in this internal Washington war took place quietly last month, when Defense Secretary Harold Brown unveiled his new "countervailing strategy" in a brief, seven-page section of the 324-page 1979 defense posture statement.

A basic part of the Brown strategy called for making the MX and Trident II ICBMs powerful and accurate enough to threaten destruction of the Soviet land-based missile force.

Until now, U.S. officials had shied away from building a strategic missile force that appeared capable of carrying out a first strike against the more than 1,500 Soviet land-based ICBMs.

Brown is, in effect, proposing a major strategic revision from the longstanding policy of mutually assured destruction. This is arms control argot for the doctrine, initiated during the mid-1960s, under which each superpower's city-destroying nuclear force could survive a first strike and still inflict unacceptable damage to the enemy.

In his statement, Brown said that such a strategy alone was no longer satisfactory. His declaration has already created an anxious stir in the arms control community.

These feelings have not surfaced publicly because there were reassurances from the White House that the president had yet to take action on the entire strategic program.

A second bureaucratic shot was fired over the weekend. It came in a published report attributed to "defense officials" that Brown spoke for the president because his posture report had prior White House approval.

A White House source said yesterday that Brown's strategy statement "was not approved" by the White House, although National Security Council staff members reviewed early drafts.

On the other hand, these sources said, the president may well approve all or part of the strategic weapons building, targeting and strategy programs Brown has proposed.

In the future, Brown said, U.S. missiles "should be able to cover hard targets [such as Soviet missile silos], with at least one reliable warhead with substantial capability to destroy that target...."

Currently, Brown said, he does not have "high confidence of destroying a large percentage of Soviet missile silos and other hard targets... with ballistic missiles."

Interviews with other officials at the Defense Department and the White House over the past week confirm that present plans call for the new MX and Trident II warheads to have a "hard-target-kill capability." This means they would be able to destroy ICBMs in their hardened silos.

The move toward ever more powerful missiles is, according to Brown, part of the "future competition in strategic capabilities" that he sees as not only continuing between the United States and the Soviet Union but as "likely to become more dynamic than need be the case."

Brown said the United States would build MX and Trident II missiles in response to Soviet development of a large ICBM force that threatened the survivability of the land-based Minuteman missiles.

Once the Trident II and MX are deployed in the late 1980s, Brown said, "flxed ICBMs will have to be regarded by both [the United States and Soviet Union] as having, at best, uncertain survivability."

Sources within the sovernment said late last week that Brown was addressing several audiences with that statement.

He was speaking first and foremost to the President, who must make up his mind on these issues in advance of the SALT debate.

He was, at the same time, trying to assure the Senate that, even with SALT II, the United States was going ahead with a major new push in strategic weaponry.