WASHINGTON — The Senate turned back a third attempt to strengthen the compromise clean air bill on Wednesday, defeating a proposal supporters said would close “loopholes” in the battle against urban smog.

Opponents argued the amendment, which lost on a 53-46 vote, would burden too many small businesses with expensive pollution controls and require unnecessary federal involvement in urban air pollution plans.

The vote marked the third unsuccessful attempt by a group of senators, mainly from urban areas with the dirtiest air, to add tougher environmental controls to a compromise bill worked out between Senate leaders and the White House.

Senate Majority Leader George Mitchell, D-Maine, argued on each vote that the compromise bill already goes far beyond current federal pollution control laws and that compromises are needed to win approval in the Senate.

But Sen. John Kerry, D-Mass., one of the key sponsors of the proposal, argued the amendment would only retain requirements in current pollution laws that are necessary to clean up the air in scores of cities so they meet federal health standards.

The provisions were weakened during negotiations on the compromise legislation.

Environmentalists said they would now focus on the House, where a clean air bill is still in committee, in their attempt to strengthen curbs on smog-causing pollutants.

In defeating the Kerry amendment, “the Senate voted to shred the environmental safety net of the Clean Air Act,” said Daniel Weiss of the Sierra Club.