WOULD YOU STAY in the Social Security system if you had your choice? It's a hefty tax, larger for some people than their income tax. Your employer might want to opt out too: he has to kick in an amount equal to your contribution. Suppose you could decide to withdraw from the system and take your chances with a private pension and disability insurance. What if hundreds of thousands of working Americans did? The system would collapse. Fortunately, Congress and the Supreme Court have acted to prevent that from happening.

Employees of state and local governments were not covered by Social Security until 1950, but, in response to requests from the states, Congress made them eligible to participate on a voluntary basis. All the states except Maine, Massachusetts, Nevada and Ohio chose to join, and even in those four, many local governments elected to participate. The law allowed any government unit to withdraw from the system, with two years' notice. For years enrollments increased rapidly. Then, in the mid-'70s, when the Social Security tax was rising steeply and rumors about the stability of the system were widespread, state and local employees began having second thoughts. Within a few years, 190,000 workers had opted out and another 227,000 had notified Washington of their intention to withdraw. Their reasons included not only the cost and uncertainty of receiving benefits; they also calculated that most workers could qualify for benefits anyway simply by putting in the minimum required time in covered employment at some point in their lives.

Congress responded to this alarming exodus by closing the door. After April 1983, no group of state or local employees could withdraw from Social Security, including those who had already filed notice of their intentions to do so. The state of California challenged the law, claiming that the states had a contract with the federal government that could not be abrogated by Congress. A federal judge in California bought the argument, but last week the Supreme Court reversed unanimously.

If the withdrawal of state and local employees had been allowed to continue, the trust funds might have lost $500 million to $1 billion a year. The system, strained as it is, could not have survived a drain of that magnitude. Social Security is the nation's basic social insurance program, protecting the elderly, the disabled, the widowed and the orphaned. It cannot survive as a voluntary mechanism. Congress was prudent in barring withdrawals, and the court correctly ratified the right of the legislature to act.