Debate is supposed to begin this afternoon in the House on the controversial proposal to bring all federal and postal workers in under the Social Security system.

If the House can keep to its timetable, a final vote should come Thursday.

The proposal is part of a major Social Security financing package approved earlier by the House Ways and Means Committee. One feature of it - the one that has rocked government-saturated Washington - is the proposal for "universal coverage" that would include civil servants. They now have their now retirement system, which is much better than Social Security, providing bigger monthly benefits and much earlier eligibility.

Federal and postal unions believe they have persuaded the House leadership - and maybe even the White House - to hold off on universal coverage. Instead, they are backing a proposal by Rep. Joseph Fisher (D-Va.) that would delay the January, 1982. entry into Social Security, Fisher's plan calls for a study to be made of the feasibility and wisdom of integrating those two very complicated and different systems.

Congressional backers of the universal converage proposal say it will end the ability of federal employee to retire from government and return to the private labor force long enough to collect payments from both systems. Their strongest argument, however, is that addition of 2.6 million federal workers payments into the Social Security system would help hold down bigger Social Security tax increases for most other American workers.

Many federal and postal union leaders believe that universal coverage is inevitable. But they don't want Congress ordering it until everybody involved understands how it would work, and federal workers are guaranteed that they will not lose any of the benefits they have worked and paid for.

Government employees pay more into their civil service retirement fund than do persons under Social Security. Their pensions are also taxable once they have recovered all the money they paid into the CS fund (usually within 18 months of retirement) whereas Social Security benefits are not taxable.

The systems are just too different, many people believe, to put them together without more information. The debate today, and Thursday's vote, will tell which way the two systems are headed.