The Maryland Senate gave preliminary approval to Gov. Robert L. Ehrlich Jr.'s slot machines bill yesterday -- but not before weakening Baltimore's ability to insist the state pay for road improvements around Pimlico Race Course.

Mayor Martin O'Malley said the one-word amendment was serious enough that it would force him to oppose the bill.

"There's a huge cost there on infrastructure, and the city cannot be put in the position of paying these costs," O'Malley said.

Before clearing the bill for a final vote today, senators turned back more than a dozen hostile amendments offered by gambling opponents.

None gathered more than 18 votes in the 47-member chamber, except for those agreed to by proponents.

Senate President Thomas V. Mike Miller, whose iron-fisted control of his chamber was evident in yesterday's roll calls, said after yesterday's session that "we're a long way from home."

"We've got to pass the bill," the Prince George's County Democrat said. "Hopefully, cooler heads will prevail in both chambers and on the second floor" -- a reference to the governor's office.

The most significant amendment to win approval -- the one on local transportation costs -- slipped through without discussion after the bill's floor leader said he would accept the change.

The seemingly innocuous change, proposed by Sen. Thomas M. Middleton, affected a clause saying: "The state shall pay for the reasonable transportation costs to ... mitigate the impact on the communities in the immediate proximity to the [slots] facility."

Middleton's amendment changed the "shall" to "may" -- making the spending discretionary for the state instead of obligatory.

None of the six city senators appeared to recognize the impact of the amendment -- which was approved without dissent.

"That 'may' doesn't do a thing," Sen. Nathaniel J. McFadden said afterward.

But Middleton, chairman of the Finance Committee, said there is a significant difference.

"The 'shall' is a mandate. 'May' is an encouragement," said the Charles County Democrat, who opposes expanded gambling.

Middleton expressed concern that the "shall" clause would force the state to give racetrack-related transportation projects priority over those in other parts of Maryland.

Sen. Brian E. Frosh, a lawyer who heads the Judicial Proceedings Committee, said "shall" could give a jurisdiction grounds to sue if the state didn't cover certain costs, but "may" likely would not. The Montgomery County Democrat said that from his point of view, the change improves the bill -- which he opposes.

O'Malley said he was "disappointed" that city senators didn't protest the change.

"Something like this would cause me to become very, very opposed," the mayor said.

His administration has estimated that a 3,000-slot machine racetrack casino at Pimlico would cost the city an additional $65 million for transportation infrastructure. The version of the bill up for a vote today by the Senate would allow an even larger facility with 3,500 machines.

O'Malley predicted that the change could cause several county executives to join him in fighting the bill. The amendment would directly affect Anne Arundel and Howard counties, which would both feel the impact of slots at Laurel Park, as well as Prince George's and Allegany counties.

Objections from local governments could raise last-minute problems for a bill that appeared to be heading for passage in the Senate.

Sen. Roy P. Dyson, a leading opponent of expanded gambling, predicted 21 no votes -- three short of the number needed to block the bill.

The measure faces a tougher reception in the House, which has passed a bill setting up a commission to study the issue and report back next year.

Speaker Michael E. Busch reiterated yesterday that he has no interest in going any further this year.

The Senate debate yesterday lasted about two hours as opponents tried unsuccessfully to hang "killer amendments" on the bill.

Dyson, a St. Mary's County Democrat, opened the attack with an attempt to convert the bill to a constitutional amendment -- putting it on the 2004 ballot to be decided by voters.

Sen. Edward J. Kasemeyer, Miller's floor leader, countered that making such decision is what legislators are sent to Annapolis to do. The Howard County Democrat said that delaying the decision until next year could mean the state would see no slots revenue until 2006.

"We need this revenue as soon as possible," Kasemeyer said, adding that the state is facing a potential $2 billion shortfall over the next several years. The amendment was defeated, 29-17.

Sen. Paul G. Pinsky took the next shot, offering an amendment cutting the racetrack owners' share of slots proceeds from the 39 percent in the revised bill to 21 percent while raising the cut for education to the 64 percent Ehrlich initially proposed.

"Mr. Governor, I think you're right," the Prince George's County Democrat said mockingly.

Pinsky estimated that the racetracks would make a combined $1.6 billion over 15 years under the Senate bill.

Kasemeyer replied that Ehrlich has concluded that the numbers in his original bill were not workable. The Pinsky amendment lost by a 30-15 vote.

Pinsky did win approval of one amendment after Kasemeyer agreed to accept language prohibiting slot machines from accepting credit and debit cards.