The MX missile cemented onto the Democratic Party's platform is President Carter's answer to Soviet blockbuster missiles aimed at the United States.

The MX, according to its backers, would have two big advantages: it would be hard to hit and it would threaten Soviet missiles the same way they threaten ours.

Faced with the prospect of losing the backbone of their nuclear offense, the Pentagon theorizes, the Soviets would be more willing "to negotiate down" -- to limit the blockbuster missiles Washington and Moscow aim at each other.

But making MX hard to hit is proving the most difficult part of the giant's birth. No one wants the MX in his backyard. And the U.S. Air Force needs lots of backyards for it.

This is because 200 missiles would be hauled among 4,600 cement garages spaced 1 1/2 miles apart. With each MX having 23 garages to hide in, in a war, Soviet gunners would have to assign two warheads to each garage to make sure they could destroy the one concealing the missile.

Since the Unite States could build MX garages more easily and cheaply than the Soviets could deploy warheads, goes the Air Force argument, the Kremlin would give up on the idea of a surprise attack. Enough MX missiles would survive to make such an attack a losing propostion and thus make nuclear war unthinkable, MX backers reason.

The Air Force wants to put the MX and its garages in the valleys of Nevada and Utah. But citizens there to not want to be invaded by the army of workers needed to deploy the MX. They fear schools would be overloaded; scarce water would be used up; landscapes would be scarred.

Right now, at Congress' insistence, the Air Force is studying other states in which it could deploy the MX so the burden would not fall on just Nevada and Utah. This is called split basing.

Building the missile itself is the easier part of getting MX into the U.S. nuclear offense. Each MX rocket would be powerful enough to hurl 10 individual warheads from the United States to Russia. Each warhead would be 335 kilotons -- 25 times more powerful than the atomic bomb which devastated Hiroshima during World War II.

The first MX missiles, under the Pentagon timetable, would be ready for duty in 1986. The cost of building and deploying the MX would run between $34 billion and $56 billion, with the Air Force giving the lower estimate and the General Accounting Office the higher one.

Critics say the MX would put nuclear war on a hair-trigger. Once both Washington and Moscow fear an accurate, powerful warhead like the MX's could destroy a missile buried under tons of concrete, goes the anti-MX argument, leaders would be tempted to fire before being fired upon.

Critics add that there is no guarantee the Soviets would give up on building enough warheads to cover every MX garage, thus accelerating rather than slowing, the arms race between the superpowers. Then the critics say, the Air Force would try to build an antiballistic-missile defense to protect the garages, making the arms race go even faster.

It is time to give up on trying to keep land missiles invulnerable, say some of the MX critics. The better idea, they contend, would be to move land missiles out to sea -- to take the cannon out of the village square.