The Maryland Senate, ignoring the threat of almost certain defeat in the House, gave preliminary approval today to a bill that would mandate the use of seat belts in the front seats of cars. Final approval is expected later this week.

Although a House committee voted last week to kill a similar bill, a point that was invoked by some senators seeking to stop the Senate version, the bill passed on an unexpectedly strong margin of 30 to 14, with three senators not voting. Sponsors of the measure said before the vote that they expected it would pass with only a bare majority.

If the law is enacted, Maryland would follow New York, New Jersey, Illinois and possibly Virginia, whose legislature is considering such a measure, in mandating the wearing of seat belts by front-seat passengers in automobiles. The state's actions have been spurred by a federal regulation requiring passive restraints such as air bags to be put in cars by 1989 unless states containing two-thirds of the country's population pass a mandatory seat belt law.

The experiences of the other states with the law were recalled today as senators engaged in a lively, and at times contentious debate on the issue, arguing freedom of choice versus the need to protect public safety.

"When it comes right down to it, people have a right to be stupid," said Sen. Julian Lapides (D-Baltimore), after reciting what he said was a quotation from philosopher Thomas Paine.

Added Sen. Jerome Connell (D-Anne Arundel), "People are sick and tired of hearing it. We continue each and every year taking away the rights of the people and the time is going to be soon when we won't have to do that anymore because there won't be any rights."

Supporters of the bill, including Sen. Howard Denis (R-Montgomery), said the state has a right to enforce restrictions like the seat belt law because people who do not wear them and are injured in accidents impose costs on their families and the rest of society.

Sen. Thomas V. Mike Miller (D-Prince George's) said the legislature should vote for the bill because it saves lives, despite what might be an immediate adverse public reaction. The measure has achieved acceptance in other states and countries as citizens saw the benefits, he said.

After the Senate gave its approval to the bill, Sen. Leo Green (D-Prince George's) offered an amendment he hoped would make the measure more palatable to the House. It would allow police officers to cite motorists for seat belt violations only after stopping them for other reasons. It was adopted by the Senate on a voice vote.

The Senate rejected an amendment offered by Sen. James Simpson (D-Charles) that would have prevented lawyers from introducing noncompliance with the law as evidence in a civil suit or as evidence of having contributed to one's own injuries in a personal injury case. In Maryland, persons who contribute to their own injuries cannot collect for damages under the state's "contributory negligence" doctrine. Miller, chairman of the Senate's Judicial Proceedings Committee, which passed the bill last month, said the amendment is unnecessary.

Despite Greene's amendment, passage of the bill is still considered slim after last week's overwhelming rejection by the House Judiciary Committee. Said committee chairman Joseph Owens (D-Montgomery), "I don't like the mandatory aspect of it . . . . We're telling people how to run their life, how to live."