If Congress imposes the so-called Medicare tax on federal workers next year, some long-service U.S. workers who have never paid into Social Security will become eligible for Medicare coverage even if they work only a couple of weeks in January and then retire.

The Senate and House this week will vote on a revenue-raising measure that contains a controversial (in the Washington area) plan to force federal employes to pay a 1.3 percent Medicare tax.

If the proposal goes into effect, federal employes will pay anywhere from $100 to $450 a year to finance the Medicare portion of Social Security. The tax would cover just about everybody in government -- from President Reagan, the Supreme Court and Congress down to the lowest rungs of the civil service.

Although the tax would pump millions of dollars into the Medicare fund, the new law would allow some government employes to become eligible for Medicare (at age 65) even if they kick in only a few dollars to Medicare and then retire.

Washington-area legislators have been trying to block the Medicare tax. They argue that it is an unfair burden on government employes who already contribute 7 percent of their annual gross income into their own retirement fund.

Many federal employes -- by virtue of work in the private sector or because a spouse was covered by Social Security -- qualify for Social Security-related benefits anyhow. But they are not now required to pay into Social Security for work performed for the government.

Senate-House conferees have approved the Medicare tax, but the entire $98 billion dollar revenue-raising measure must be voted on again -- probably Thursday or Friday -- by the full House and the full Senate.

The bill would mean a new, long-term tax bite for most government workers. But for a lucky few, who have spent their entire careers in government without paying into Social Security, it would mean an almost free ride into Medicare if they hold off retiring until the new law goes into effect.