Leaders representing 10.5 million active and retired federal workers and their families are counting on the House to kill a Senate plan that could cause a 100 percent increase in health insurance premiums next year for many present and former civil servants.

The Senate Finace Committee has okayed legislation that would make the Federal Employee Health Benefits (FEHB) plan the primary payer of medical and hospital costs for millions of retirees who now depend on Medicare to pay most of their bills. The Senate is almost certain to go along with the proposal, which is part of the budget reconciliation bill.

If the change becomes law, premiums for some plans could double in 1982 because they would have to begin paying retirees' health costs now primarily borne by Medicare. Between 50 percent and 70 percent of all federal retirees qualify for Social Security and Medicare coverage through private sector work or a spouse's Social Security eligibility.

Medicare comes into play when an individual is 65. In the case of federal and postal retirees who are covered by the FEHB program, Medicare now picks up most medical costs. The FEHB takes care of some deductibles or costs not covered by Medicare. The plan cleared by the Finance Committee (a 17 to 5 vote) is almost certain to be adopted by the full Senate. A similar measure, introduced by Rep. Charles B. Rangel (D-N.Y.), is pending before a House Ways and Means Subcommittee.

Last week, National Association of Letter Carriers lobbyist George Gould and American Postal Workers Union chief Moe Biler went to see Rangel, to explain the impact his change (the same one being considered by the Senate) would have on the FEHB. They said Rangel promptly agreed to see that his language is stricken from the bill when it comes up before the subcommittee or for a vote in Ways and Means.

If the Medicare-FEHB shift passes the Senate and is blocked by the House, it will go to a Senate-House conference committee along with other disputed items in the reconciliation packages. Lobbyists feel they have a fighting chance of blocking the change in conference.