WASHINGTON (AP) _ Eight years after Marsha Christopher was attacked by a vicious dog on her first day as a mail carrier, Congress still is trying to enact a ''private'' bill to help the Michigan woman pay her medical expenses.

On Aug. 9, 1979, a bull mastiff jumped Mrs. Christopher and tore away portions of her face as she walked her Postal Service route in the suburban Detroit community of Taylor.

Mrs. Christopher's superiors ''failed to follow prescribed warning procedures, and claimant was not informed that this dog had attacked three different mail carriers in the preceding two years,'' the House Judiciary Committee said in a report on the case last year.

Despite dozens of operations, the woman remained severely disfigured and still was under treatment by a plastic surgeon, the committee said.

Mrs. Christopher, who is from Romulus, Mich., received a $100,500 settlement from the dog's owners and paid her attorney $33,500 of that, unaware that federal law would require her to use a large portion of the damages she received to reimburse the government for its expenses on her behalf.

The government forced her to repay $26,000 of the $40,000 she had received in workers' compensation benefits and ruled that she must forego another $14,000 in future medical compensation.

Sen. Donald Riegle, D-Mich., introduced legislation Thursday that would return the money Mrs. Christopher was forced to repay the government and make her eligible for $14,000 in additional medical benefits.

The legislation, similar to one that Rep. William Ford, D-Mich., introduced earlier this year and in the past three sessions of Congress, is known as a ''private'' bill - one that singles out a specific person for relief from a problem related to the government.

Members of Congress generally introduce private bills only in the most compelling of cases, and, even then, such proposals rarely become law. In the 1985-86 session of Congress, just 24 private bills were enacted.

In the 1981-82 session, Ford's legislation to aid Mrs. Christopher died in a House committee. Two years later, a similar bill was passed by Congress but vetoed by President Reagan, based on a recommendation from the Labor Department.

Reagan wrote that while he could sympathize with Mrs. Christopher's plight, legislation to assist her ''would set an undesireable and potentially costly precedent and would discriminate unfairly against the thousands of other postal workers and federal employees who also incur job-related injuries.''

The legislation again was introduced by Ford in the last session of Congress, and hopes for its enactment rose after the Labor Department withdrew its opposition, but the bill died in the Senate after passing the House.