Mayor Bloomberg is calling on Congress to back new legislation to keep guns away from terrorism suspects.

Yesterday, the mayor and Senator Lautenberg of New Jersey, a lead sponsor of the bill, said current federal law has an inexcusable loophole that allows individuals on terrorism watch lists to purchase guns legally.

The mayor said Congress cannot wait and must act before a terrorist opens fire in a restaurant, train station, school, or other public location.

“How many more warning signs do we need?” Mr. Bloomberg said during a news conference at the base of the Brooklyn Bridge, one of the city’s most glaring terrorist targets. “We know that terrorists want guns, we know who many of them are, and we’re not doing anything about it.”

Mr. Lautenberg, a Democrat who is jointly pushing the legislation with Rep. Peter King, a Republican of Long Island, said, “There’s a gap in our laws that defies common sense.”

He cited a terrorist training manual discovered in Afghanistan in 2001 instructing would-be terrorists to buy weapons in America. He also cited a 2004 investigation, conducted at his request while he was seeking support for a prior version of the bill, which found that individuals on various terrorist watch lists had been able to buy guns from licensed dealers 47 times during a nine-month stretch.

“This isn’t a theoretical exercise,” Mr. Lautenberg said. “Terrorists cannot only buy guns, they do.”

Both men said that now that the Bush administration is on board, the legislation has a strong chance of passing. In April, the Justice Department changed course and proposed legislation that Mr. Lautenberg has formally introduced.

According to a New York Times report, a former attorney general, John Ashcroft, had blocked the FBI from cross-referencing suspects in the September 11, 2001, attacks and gun purchases.

The influential National Rifle Association, which has a reputation for holding sway over many members of Congress, said yesterday that it is against the measure.

The chief lobbyist for the NRA, Christopher Cox, said that while his group does not want terrorists to get guns, the proposal would “give authority to a political appointee to arbitrarily deny constitutional rights with no due process.”

Mr. Cox said simply being on a watch list does not translate to being a terrorist and that many people are mistakenly put on the list. He cited Senator Kennedy, a Democrat of Massachusetts, who was once prevented from boarding a plane because someone else with his name was on the list.

“It’s a secret list,” he said. “Nobody knows how you get on it, nobody knows whether they are on it or not, and there is no way to get off.”

He said there is too little due process and that the system could actually help terrorists by alerting them that they are being watched. Mr. Bloomberg dismissed that claim and said that only groups that don’t support it are “special interest extremists.”

The mayor has been in an escalating battle with the NRA over gun control measures since the beginning of his second term.