WASHINGTON — When handguns with bullets that can pierce body armor showed up on the streets of New Jersey, Senator Lautenberg asked federal regulators to share data that could help local police figure out where the weapons were coming from. That information, the New Jersey Democrat was told, is off-limits.

The amendment that bars such sharing of gun trace data has now touched off a feud between its sponsor, Rep. Todd Tiahrt, a Republican of Kansas, and a coalition of more than 200 mayors led by Mayor Bloomberg and Boston Mayor Thomas Menino.

Insisting that gun trace data is an essential crime-fighting tool for cities, Mr. Bloomberg used his own funds as seed money, formed Mayors Against Illegal Guns and made repealing Mr. Tiahrt’s amendment its number one issue this year.

Messrs. Tiahrt and Bloomberg met in Washington earlier this year to try to find a compromise. “I thought we were close,” Mr. Tiahrt said. But when Mr. Tiahrt showed his proposed changes, “it was one step forward, three steps back,” said Mr. Bloomberg’s criminal justice coordinator, John Feinblatt.

So last week, the mayors’ group began airing television ads against Mr. Tiahrt’s amendment in several broadcast markets, including Mr. Tiahrt’s Wichita district — a step Mr. Tiahrt said went too far.

“I think that personal attack is something that’s very upsetting to me and very unjustified and a great mischaracterization of all my efforts here,” Mr. Tiahrt said.

Two TV stations in Wichita declined to air the ads, saying they could not verify the claims.

Mr. Tiahrt said his amendment, first passed in 2003 and typically attached to a major spending bill each year, is meant to protect sensitive information that, if shared with local police, could jeopardize active crime investigations.

Besides, he said, the Bureau of Alcohol, Tobacco, Firearms, and Explosives has misinterpreted the amendment and tends to withhold more information than necessary.

Mr. Bloomberg’s aide countered that the amendment “makes police officers do their job with a blindfold on.”

“What you want to know is, for my jurisdiction, are there a small number of dealers who keep supplying crime guns to my city? And you can’t get that information,” Mr. Feinblatt said.

Under the Mr. Tiahrt amendment, local law enforcement agencies can obtain gun trace data from ATF if it concerns a specific criminal investigation or prosecution. But many police chiefs, mayors and their supporters in Congress say cities need access to ATF’s broader data to help fight crime.

In New Jersey’s case, Mr. Lautenberg said, Mr. Tiahrt’s amendment made it harder for police to identify trends in illegal gun trafficking, the types of guns criminals are getting, and which gun shops provided the weapons.

“We need to be able to identify where guns come from to combat gun violence and protect our communities,” Mr. Lautenberg said.