Big Labor is back in Congress this year with a piece of legislation known as the ''common situs'' or ''double-breasting'' bill. The construction unions managed to get it through Congress in 1976; President Ford killed it with a veto. The unions tried again in 1977; the bill failed in the House. They tried once more in 1986; Orrin Hatch of Utah managed to stop it in the Senate. Now the bill has passed the House (by a vote of 227-197 on June 17) and has cleared a Senate subcommittee. A Senate version is poised to go to the floor.

The bill is flatly indefensible. It bears a fraudulent title: ostensibly it is a bill ''to increase the stability of collective bargaining in the building and construction industry.'' On the contrary, the bill would destroy a delicately balanced system of labor relations that has worked for many years. It would dragoon millions of nonunion construction workers into unions against their will. It would result in staggering increases in construction costs. The public would suffer, but what else is new? These unions operate on a rule established by the robber barons a century ago: the public be damned.

In theory, the legislation is intended to close a loophole in labor law by which construction companies get away with ''double-breasting.'' This is a practice by which a single employer creates two or more subsidiaries, one of which is union, the others nonunion. Most of these corporate structures are entirely legitimate. A few of them -- no one denies it -- are sham creations, set up to evade collective bargaining responsibilities.

No such remedial legislation is needed. The problem is not pervasive. Under existing law, the National Labor Relations Board may determine that the shams constitute an unfair labor practice. The board has not hesitated to use its authority.

What would the pending Senate bill do? It would effectively apply to virtually every construction company in the nation with even one unionized operation. By a tricky redefinition of the term ''single employer,'' the bill would compel every such contractor to impose a union shop on all his subsidiary workers. The bill would rewrite the existing law on prehire union halls so that certification or decertification elections (if any were held) could be stacked and rigged in the unions' favor.

The faceless construction workers -- the nonunion carpenters, steamfitters, electricians, plumbers, bricklayers -- would have no opportunity to vote yea or nay on whether they wished to join the union. None of the protective provisions of existing law would apply. And watch the union dues roll in!

Notice, if you please, the brazen terms of the Senate bill. It would embrace ''any two or more business entities'' performing the same ''or similar'' work in the same ''or in different geographical areas.'' This is a finely woven net. All that is required is that these entities, ''directly or indirectly,'' have ''substantial common ownership; common management; or common control.''

Grammarians will note the exquisite care with which that definition is composed. In the Senate bill the adjective ''substantial'' modifies only ''common ownership.'' Any degree of common management or ''control'' would be sufficient to trigger the press-gang provisions. In the bill that passed the House, this section was rewritten so that ''substantial'' applies uniformly; the language about ''the same or different geographi-cal areas'' was made a little more obscure, but the basic purpose ofthe bill, to butter up the building trades, has not been altered in the slightest.

It is easy to understand the hunger of the construction unions. Forty or 50 years ago the industry was highly unionized. Then the unions began to self-destruct. They demanded and won wage scales far above the average. Of greater importance, they began to impose work rules that limited productivity and boosted costs. Contractors began to fight back. By 1973 roughly 40 percent of building workers were union members. Today it is estimated that only 25 percent belong.

Thus this devious legislation. The House amendments notwithstanding, it is the same old bill that lovers of individual liberty have been fighting for so long.