Federal workers who expect to leave the government, who have a child about to turn age 22 or who have a dependent considering marriage need to shop carefully during the health insurance open season that ends Dec. 11.

If you or a family member is likely to lose eligibility to belong to or be covered by the federal group insurance program, you should make certain that the 1988 plan has conversion rights that will allow you (or your dependent) to purchase comparable health coverage outside of government.

If you think your current federal health premium is high, wait until you try to buy a similar nongroup policy on the outside. Chances are your premiums will at least double, and if you have special medical problems you could have trouble finding a plan that will take you.

Uncle Sam requires that all health plans participating in the federal health program guarantee so-called conversion rights for you. Generally speaking, your children cannot be covered by your health plan after they reach age 22 (or if they marry) unless they are severely handicapped.

Those conversion rights allow you to enroll in a plan sponsored by the same group, organization or company. But because the government won't be paying a portion of your premiums (typically it pays about 60 percent), and because you may be charged single -- rather than group -- rates, conversion will likely be expensive.

Washington Consumers Checkbook's guide to federal health plans recommends that employes who think that they may be leaving government in 1988 look at Aetna and Blue Cross-Blue Shield, which have the conversion option. Checkbook says that many health maintenance organizations offer the conversion option, as do most -- but not all -- of the plans sponsored by unions and organizations.

But Checkbook advises you to read the brochure of each plan carefully to determine how much you will have to pay and to see if the coverage is similar.

For people whose children will turn age 22 (and no longer be eligible for coverage under your family plan), Checkbook says to consider these plans, which offer "excellent" conversion benefits: The National Alliance of Postal and Federal Employees plan, Foreign Service plan, NATA plan, National Association of Government Employees, Postal Supervisors and SAMBA. Those plans, Checkbook says, allow children to continue in the high-option coverage at one-half the usual cost for single employes. Because there will be no government contribution, you will pay substantially higher rates. Hatch Act

The House is due to take up legislation today that would amend the Hatch "no politics" Act to allow U.S. workers to take active roles in partisan political campaigns. Under the bill, sponsored by Rep. William Clay (D-Mo.), civil servants could manage political campaigns, help raise funds or even run for office as partisan candidates while off duty or on leave.

Backers of Hatch Act liberalization claim that the 48-year-old law has made federal workers second-class citizens and deprived the nation's political process of the input of 3 million of its best-educated, best-paid and best-informed people. Opponents of any change fear that relaxing the act would make it easier for bosses to lean on employes for political contributions or political chores.