WASHINGTON (AP) _ President Bush, weighing an issue that could alter the basis of nuclear negotiations, has postponed a decision on whether to continue building MX missiles or switch to the smaller, single-warhead Midgetman, his spokesman said Tuesday.

White House press secretary Marlin Fitzwater said Bush will not meet a Wednesday deadline imposed by Congress for a decision.

''We are reviewing the entire strategic posture of the country and it is logical and necessary that we postpone this major decision until the review is complete,'' Fitzwater said.

Because of the review, the administration also has put off resuming nuclear arms control talks with the Soviet Union. They were to have started anew in Geneva on Wednesday.

In an era of tight budgets and big deficits, it is generally agreed the Pentagon cannot build both the 10-warhead MX missile and the single-warhead Midgetman.

Trying to force a decision from the White House, Congress last year stipulated that $350 million which was earmarked for the MX program could not be spent until the new administration announced how it would proceed.

Brent Scowcroft, the president's national security adviser, has argued for years that a single-warhead missile such as the Midgetman - weighing about 15 tons as opposed to 100 tons for the MX - could ease a Soviet threat to U.S. land-based missiles in fixed silos.

A smaller missile with only one warhead would be a far less tempting target to Soviet war planners, it is reasoned, and could be the basis for arms control agreements based on counting the number of warheads, not launchers.

Currently, the number of launchers is the key element in arms control agreements. That has spurred development of multiple warhead weapons, such as the triple warhead Minuteman and the 10-warhead MX and similar weapons in the Soviet arsenal.

In 1983, Scowcroft was head of a presidential commission on strategic forces during the Reagan administration and advocated development of the Midgetman. He has not publicly endorsed either the MX or the Midgetman since joining Bush's administration.

Fitzwater said the White House will send a report to Congress on Wednesday explaining that the administration is conducting a review but omitting any decision regarding the MX or Midgetman or other type of small intercontinental ballistic missiles.

He said he could not estimate when the White House would reach a decision but noted that Bush has instructed advisers to report within 90 days on the overall review of national security policy.

The spokesman said that the stalled confirmation process surrounding John Tower's nomination as secretary of defense had not been a factor in delaying the missile decision. ''He's in on much of the review,'' Fitzwater said.