Federal and postal workers will begin paying a Medicare tax in January - from $108 to $450 per year depending on salary - unless friendly legislators and lobbyists can persuade the Democratic House to lock horns with the Republican Senate on the explosive political issue.

The Senate Finance Committee and House Ways and Means Committee, which write Social Security legislation, have approved the 1.3 percent Medicare tax for federal workers. If it becomes law the deductions, which would come out of every paycheck for the average civil servant, would begin in January 1983.

It would be the first time that federal workers - by virtue of their U.S. jobs - paid into the Social Security system.

It would be the first step toward bringing the government's 2.8 million employes into the Social Security program.

Opponents of the Medicare tax for federal employes had hoped to get the House committee this week to drop the proposal from the reconciliation package - covering all of Social Security - it has approved. The committee has decided against any changes.

This means that if the Medicare tax is to be eliminated from the House version of the budget reconcilliation bill it must be taken out on the House floor.

Many members of Congress, most of them Democrats, don't like the Medicare idea.

Whether they will stand up and oppose it, and vote against it, is another matter.

The Medicare tax idea is most unpopular here, where most voters work for the federal government, but away from Washington the average voter probably thinks it is a good idea to make federal employes join the Social Security club.

If the House decides to drop the Medicare tax - and that is IF with capital letters - the matter then would be taken up by Senate-House conferees, and the Senate members would have to agree to drop it from their version.