The members of the Senate Appropriations Committee are scheduled to vote today on the Tiahrt amendment, a piece of legislation that has been in the crosshairs of Mayor Bloomberg since the beginning of his campaign against illegal guns in 2006.

Lobbyists for and against the amendment said in interviews that they were scrambling to talk to lawmakers and their aides as the vote approached.

“This is our no. 1 legislative priority on Capitol Hill,” the executive director of the National Rifle Association’s Institute of Legislative Affairs, Christopher Cox, said. “I’ve been running around like crazy.”

The president of the Brady Campaign Against Gun Violence, Paul Helmke, said, “It’s the no. 1 thing we’re pushing on at this stage, too.”

The amendment is named after Todd Tiahrt, a Republican of Kansas, who introduced it in 2003 as a rider on the Bureau of Alcohol, Tobacco, Firearms, and Explosives appropriations bill. The goal, he says, is to restrict municipalities from using firearm trace data to file civil lawsuits against gun dealers. Mr. Bloomberg contends that the amendment restricts law enforcement personnel from getting data that could help them shut down crooked gun dealers.

Mr. Helmke said he expected Senator Shelby, a Republican of Alabama, to introduce an amendment that would make the Tiahrt amendment even stronger.

“We are expecting this amendment could be even worse than in previous years,” Mr. Shelby said. “That would lead to more paperwork for law enforcement and increased liability.”

He said that he believed Senator Lautenberg, a Democrat of New Jersey, would try to reword Mr. Shelby’s amendment to make it “less bad.”

The vote will be close, observers said.

Democrats have a slight majority in the committee, but not all Democrats support gun control. For instance, Senator Johnson, a Democrat of South Dakota, represents a state with a largely pro-gun stance and is not expected to vote against the amendment.

The House Appropriations Committee is set to vote on the amendment in the second week of July. The speaker, Rep. Nancy Pelosi, is against the amendment, a spokesman, Nadeam Elshami, said.

Mr. Bloomberg and the coalition he cofounded, Mayors Against Illegal Guns, have spent the last months airing television advertisements against the amendment and lobbying legislators. The NRA has responded by sending letters to television stations that run the coalition’s advertisements to dissuade them from broadcasting what they describe as factually incorrect information. Some stations have refused to run the segments.

“Gun owners in this country do not believe that the mayor’s elitist mind-set … should be pushed on the rest of America,” Mr. Cox, the NRA lobbyist, said.

Though dozens of mayors have joined Mayors Against Illegal Guns in the last several months, there has also been a trickle of mayors leaving the coalition. The mayor of Carmel, Ind., James Brainard, said he left the coalition this month when “the organization’s mission began to expand and was no longer reflective of my constituency.”