IF YOU thought that the statesmanlike behavior that has guided congressional consideration of the Social Security reform package was too uncharacteristic to last, you may well have been right--we shall see. Sen. Russell Long is sponsoring a floor amendment to the package that could upset the whole compromise.

Sen. Long, and those who support him, have succumbed to a seemingly innocuous contention by government employees' unions. The unions argue that if new federal workers are going to have to be covered by Social Security--as called for by the reform measures passed by the House and reported by the Senate Finance Committee--that action should be delayed until a supplementary pension system has been passed by Congress.

The government unions have been spending millions of dollars trying to persuade the public and Congress that federal workers shouldn't be covered by Social Security at all. Thus far, Congress has been smart enough to see through the unions' widely advertised contention that the move will cost taxpayers billions of dollars a year. After all, if that were so, it could only be because the workers were going to get better benefits. And if that were the case, why would they be complaining?

The truth, of course, is that the taxpayer is already paying most of the cost of the generous federal retirement system. By eliminating the expensive windfall Social Security benefits that many federal retirees can now receive in addition to their pensions, it is possible to provide both regular Social Security coverage and a fully adequate supplementary pension and still save billions.

Federal workers deserve a supplementary pension comparable to a good private sector plan. But since only new workers will be covered, it will be many years before any of them qualify for benefits. Several sensible alternatives were laid out a few years ago by a study commission set up by Congress in 1977, and a particularly attractive version was developed last year by the Senate civil service subcommittee. With this work already done, Congress could move swiftly to put a new system in place.

But delaying Social Security coverage until a supplementary plan is put into law would almost surely mean that the change would never happen. Without the cover provided by the complete Social Security reform package, it is most unlikely that Congress would have the courage to stand up to the goverment workers' lobby and put a new pension package into law. And if federal workers are exempted from the Social Security measure, support for curtailing benefits for far less favored groups of retirees will be greatly weakened.

That's why congressional leaders from both parties, who have worked long and hard to develop a sound Social Security rescue package, correctly view Sen. Long's amendment as very dangerous. The Senate must not cave in to the government workers' lobby on this key vote.