A mandatory seat-belt bill was approved by one vote last night in a Senate committee, a sign of the difficulty it faces getting through the legislature.

By a 6-to-5 margin, the Judicial Proceedings Committee voted to require people who sit next to a window in the front seat of a car or multipurpose vehicle to wear seat belts or face a maximum fine of $20.

The proposed law would exclude some persons, including postal and newspaper carriers, as well as taxicab drivers. A provision also was included for medical exemptions.

U.S. Transportation Secretary Elizabeth Dole told car makers last year that they would have to install air bags or automatic seat belts unless states representing two-thirds of the population pass mandatory belt-use laws, carrying a minimum fine of $25, by 1989.

Sen. Gerald Winegrad (D-Anne Arundel), a member of the committee, urged members to keep the fine below $25 so that car makers could not get out of additional safety requirements.

Sen. Thomas V. Mike Miller Jr. (D-Prince George's), the committee chairman and a cosponsor of the bill, said he expected the legislation would clear the Senate. If so, it faces an uphill battle in the House Judiciary Committee, chaired by Del. Joseph Owens (D-Montgomery), a staunch opponent of a mandatory seat-belt law.

Only New Jersey, New York and Illinois have enacted laws requiring use of seat belts.