The Maryland Senate voted by a lopsided margin today to repeal the state's unique ban on the mining of steep coal fields, rejecting a liberal member's plea to "have the guts" to preserve that decade-old prohibition.

Lobbyists for the Maryland Coal Association and legislators from Western Maryland buttonholed senators before the 29-to-14 vote on second reading, which all but assures the Senate's final passage of the bill later this week. Those proponents, who have lobbied hard for the bill they regard as economic balm for Maryland's ailing coal industry, said they were delighted by the outcome.

Opponents, who included several suburban Maryland senators and a small band of conservation groups, said they would try to defeat the bill in the House, where a committee is now considering virtually identical legislation.

John N. Bambacus, the first-term Republican senator from Allegeny County who championed the repeal of the steep-slope law, said the Senate vote proved "what can be done if you take the emotions and politics out the debate."

But during today's floor debate, Bambacus himself made a brief, emotional appeal to his colleagues, saying, "I am a Western Maryland coal spokesman and I'm proud to be the spokesman."

Bambacus, who has been harshly criticized by some in his district for receiving large contributions from coal companies in his 1982 campaign, said the repeal would make local coal companies more competitive in a slumping market and preserve the estimated 1,000 coal-related jobs in Allegeny and Garrett counties.

Bambacus also stressed that amendments he wrote and included in the bill would go far towards protecting the environment in his rural district. The amendments, adopted last week, set forth five tests a mining company would have to pass before winning a permit to mine slopes steeper than 20 degrees.

But Sen. Julian L. Lapides (D-Baltimore City), an opponent of all strip-mining and the lone speaker today against lifting the steep-slope ban, called the repeal bill "a classic example of the vested interests on one side and the people's interest on the other side."

Before the vote, Lapides suggested that the expensive lobbying effort mounted by the Maryland Coal Association to ensure the bill's passage would be the key factor in the Senate's almost certain approval of the bill.

Lapides derided a coal industry lobbyist for telling a reporter weeks ago that the repeal bill would be "finished business" in the General Assembly.

"Your vote was already counted," said Lapides, gesturing around the ornate Senate chamber. He added: "The people who came down by the busload on a bitter cold day to testify against the bill don't have money to hire an expensive lobbyist."

But Sen. Norman R. Stone Jr. (D-Baltimore County), chairman of the key committee that approved the bill on an 8-to-3 vote last week, countered by saying of the coal industry, "There's nothing evil about it."

The disruption of parts of the Appalachians in Western Maryland is "the nature of the industry that employs our people and keeps our state viable," Stone said.