Objections by western governors to the amount of their land needed for the proposed cement trench system to base the planned 200 new MX mobile ICBMs has led the Pentagon to abandon that idea and propose another, according to Defense Department officials.

Last month, President Carter announced his decision to build and deploy the $30 billion-plus land-based intercontinental missile system and make it mobile so it would not be vulnerable to a Soviet first-strike attack.

But finding a basing mode that would hide the giant MX missiles, yet still make them available for the Soviets to count under SALT II provisions, has proved difficult.

The task has been further complicated by a need to meet both the political and environmental desires of the residents of Nevada, Utah, Arizona and New Mexico, where the missiles are to be located.

The newest basing favorite, according to one Defense official, seems to satisfy everyone's needs, though it has emerged only in the past month after earlier proposals fell by the wayside.

As how envisioned, each MX missile would ride on a transporter-launcher on a race track-shaped roadway that would have 20 to 25 protective cement shelters on side roads, or spurs, some 1.5 miles apart.

The race tracks - probably in groups of four - would be located in valleys, primarily on government-owned land.

The missile on its transporter would move from shelter to shelter in an irregular pattern and would have the ability to "dash" from one shelter to a distant one if there was warning of an attack.

The roofs of the shelters could be pulled back to show that there was only one missile on each race track.

The only fenced-off portion of the new system would be the 2 to 2.1/2 acres around each cement shelter that might, or might not, contain the traveling MX missile.

A Defense official said yesterday that the main circular road on which the missile and its transporter would travel "would be open to the public, to recreational vehicles, campers and others" and could be used "to connect the extremities of the valleys."

When the missile and transporter were moving, he added, they would be covered by a "shield vehicle." And when the missile and transporter were inside a shelter, the "shield vehicle" would be parked, so the MX system would be, he said, "spy-proof and the Soviets could never tell where the real missile was."

The official said he hoped a Presidential Policy Review Committee would approve the basing system next week so that Defense Secretary Harold Brown could take it to the White House.

An MX basing decision by Carter has been forecast for Aug. 1, according to congressional sources.

Selection of the big MX missile and establishment of a basing plan have been considered important in helping the administration get approval for SALT II.

Carter is seeking about $1 billion from Congress in fiscal 1980 funds for MX research and development. And although a final decision on a basing system is not necessary for another year, setting up a program would reassure senators concerned that MX support, like that for the B1 bomber, may be only temporary.

MX basing studies have gone on for years, and several plans have been publicly promoted only to be dropped.

The first idea was called the "shell game" - take one MX missile and move it cladestinely among 20 holes in the ground. That was ruled out by the White House because U.S. intelligence officials said if the Soviets adopted the same system, the United States could not be certain the Soviets had only one missile in such a field.

In May, the Pentagon hastily turned to the trench notion - running one missile between 40 launch sites on tracks on a 20-mile long trench. It was the favorite system when on June 8m Carter declared he would go ahead with the big new land-based missile system.