The House Rules Committee - the all-important congressional traffic cop - makes a crucial, multibillion dollar decision on Tuesday.

At issue is whether federal and postal workers are brought under the Social Security blanket, and how much more people already in the system will have to pay to finance government-backed old-age pensions.

Backers of the plan to extend mandatory Social Security coverage to 2.6 million federal workers hope the committee will vote for a "closed rule" on the legislative package already cleared by the Ways and Means Committee. A closed rule means the bill would go to the House floor as is, thereby requiring members to vote yes or no on the bill - including the provision for mandatory Social Security coverage for government employees.

Opponents of "universal coverage" for federal workers - who now have their own attractive retirement system - want a "modified rule" from the committee. That would permit as many as eight already-agreed to amendments to be presented to the full House. One of them, by Rep. Joseph Fisher (D-Va.) would at least temporarily block mandatory coverage for federal workers.

Earlier, the Ways and Means Committee voted to extend Social Security coverage to federal workers by 1932. Fisher's amendment lost in committee, but he did get it to approve language requiring a study - results due by 1930 - on the impact of meshing the two very different systems. It includes language to protect the rights and benefits of federal workers when they retire.

Few people pretend to understand the complicated implications of the Social Security-CS pension linkup. But both sides predict varying degrees of financial disaster for the nation if they lose.

The pre-universal coverage people say that putting everybody under Social Security would protect wage-earners from substantially bigger pay-check bites in the future to finance obligations of the system. They also argue that it would close legal loop-holes that permit federal workers to retire early from the government and work for minimum periods under Social Security to get benefits from it too. They say this is a form of "double dipping" that is widely practiced by government workers and not generally available to people who don't work for the government.

People against universal coverage - and this includes most federal and postal unions - say there are too many unanswered questions about the incredibly complex problems of blending the two systems. They say the Ways and Means Committee is trying to drag them under Social Security to pump money into the financially troubled system that provides basic retirement benefits to most American workers.

What Ways and Means is doing, they say, is trying to duck the politically unpopular option of raising Social Security taxes if government workers are not brought into the Social Security system.

Many federal workers - and retirees - are frantic over the prospect of universal coverage. They believe they could wind up paying paying much more for the same, or even reduced, retirement benefits. They also fear that it could eventually result in Congress deliberalizing federal retirement rules that permit government workers to retire earlier than most private industry workers.

Federal employees pay more into their retirement fund than do people under Social Security and federal pension benefits are taxable, while Social Security payments are not. The only thing both sides agree on is that the problem of universal coverage is complex, because the two systems are so different.

There has been tremendous lobying in Congress and in districts back home both for and against the issue of universal coverage.

People who want it hope the Rules Committee will vote for a "closed rule" so that the following House vote is simplier, and will be on the issue they prefer.

People opposing universal coverage want a "modified rule" so that the Fisher amendment will get a test on the House floor.

AFL-CIO unions representing federal and public employees are split on the issue. Generally, groups representing federal workers, postal employees and teachers and firemen are against universal coverage - at least until a study explaining what it is made.

The big American Federation of State, County and Municipal Employees believes that universal coverage is inevitable - and good since it will help the unfunded liabilities of state governments who, AFSCME says, face serious if not impossible problems in paying off pension committments to workers outside the social security system.