The Pentagon has formally committed itself to rushing the MX mobile missile toward production by promising congressional leaders to go into fullscale development this year.

Defense Secretary Harold Brown, in recent conversation with congressional leaders, has said the need for the missile is no longer in dispute but the administration still wants until April 1 to decide whether to put it inside airplanes or in missile fields.

President Carter intends to request $250 million in fiscal 1979 supplemental funds from the new Congress for the MX and $750 million in the fiscal 1980 budget now in the final stages of preparation.

The MX represents one of several significant steps being taken in hopes of making land-based missiles less vulnerable to the increasingly accurate warheads both the United States and Soviet Union are deploying.

The warheads of the 1980s will be so accurate that anything stationary -- like the 1,054 U.S. intercontinental ballistic missiles (ICBM) standing in concrete, underground silos -- would probably be destroyed, according to Pentagon analysts.

The MX will be mobile to make it hard to hit. The Air Force wants to haul each nuclear blockbuster around a field of about 20 identical silos, secretly inserting the missile periodically in different ones so Soviet gunners would not where it was.

The other leading idea is to put the MX in special airplanes that could take off in a hurry from a short runway to escape surprise attack.

Another way of protecting land-based missiles is to go back to antiballistic missiles -- the attempt to hit a bullet with a bullet. This idea has fresh appeal, according to a new government study.

"Pressures are increasing for the superpowers to consider defensive solutions to the increasing vulnerability of land-based missiles," said the Library of Congress in a report recently issued by the House International Relations Committee.

Part of the pressure to go back to the ABM -- which the United States spent $5 billion on before abandoning it -- is coming from both the growing realization that stationary missiles are in danger, no matter how much concrete is protecting them, and the recent progress in developing better antiballistic missiles, the report said.

Administration officials were citing Carter's support of going ahead with full-scale development of the MX, the last step before starting production, as hard evidence he has no intention of abandoning land-based missiles anytime soon.

An MX system would not be deployed until around 1986 at a cost of between $20 billion and $40 billion. The program could be called off at any stage, however.

A column by Robert Novak and Rowland Evans yesterday said that "the gloomy consensus" at the Pentagon is that Carter "is determined, for the sake of arms control, to reject the Defense Department's proposals to preserve the land-based ICBM and the Triad" -- the term for the triple strategic offense of land-based ICBM, submarine-based ICBM, and longrange bombers.

The column also said that Defense Secretary Brown had warned Carter in a top secret memo Dec. 1 that giving up land-based missiles would have "disastrous consequences both internationally and domestically."

Pentagon spokesman Thomas B. Ross said yesterday that the column was "wrong" in the quotations attributed to Brown and the date he sent it to the White House. Ross did not deny Brown had expressed such concerns, however.

Ross also denied the column's assertion that Brown favors the "shell game" type of approach for MX. Ross said Brown has not yet made up his mind.