The House Post Office-Civil Service Committee began hearings yesterday on a Democratic proposal to set up a new retirement program for federal workers hired since January 1984.

It provides better civil service benefits (but less generous tax-deferred investment options) than two proposals cleared earlier by the Senate Governmental Affairs Committee. The Senate and House plans would be mandatory for workers hired since January 1984 who are covered by Social Security. No employe covered by the current civil service retirement system would be required to join any new plan approved by Congress.

The two Senate retirement plans would allow longtime federal workers to switch into either option. The House bill covers only post-1983 hires.

All three options base retirement benefits on a combination of civil service, Social Security and earnings from thrift plan investments. The Senate options provide smaller civil service benefits than under the current system.

The House bill, by Reps. William Ford (D-Mich.) and Mary Rose Oakar (D-Ohio), would give future retirees a bigger civil service pension than either of the Senate plans, but a less generous tax-deferred investment option.

Federal and postal union leaders prefer the Democratic plan. They say the Senate options are designed for higher-income employes and do not give enough basic pension benefit guarantees to middle- and lower-income workers.

The Senate and House are expected to approve their respective plans this month. Then the bills will go to a Senate-House conference committee. It will try to work out a compromise for the 300,000 feds hired since January of 1984.