WASHINGTON — Taking his fight against illegal guns to Capitol Hill, Mayor Bloomberg has a warning for members of the new Congress: Vote for a measure backed by the gun lobby, and he’ll remember in 2008.

Mr. Bloomberg issued his threat yesterday as he convened a national summit of more than 50 fellow mayors from both major parties, all members of the coalition he cofounded to combat illegal guns. The day-long session, which included leaders from Los Angeles, Chicago, and Newark, centered on ways to advance federal legislation to help police track illegal gun sales, and to oppose measures that restrict law enforcement.

The mayors zeroed in on a measure known as the Tiahrt Amendment, named after its chief sponsor, Rep. Todd Tiahrt, a Republican of Kansas. The provision would prevent the police from using some gun sales records from the Bureau of Alcohol, Tobacco, Firearms, and Explosives. Versions of the amendment have passed Congress for the last four years after being attached to larger Department of Justice appropriations bills.

With the backing of the National Rifle Association, some lawmakers have pushed for making the restrictions permanent, a proposal the coalition yesterday vowed to oppose.

“This is a law that restricts information that is there and that any rational person would want in the hands of any rational person in this country,” Mr. Bloomberg said.

Because the amendment is often attached to much larger bills, members may defend their approval of it as a compromise vote aimed at passing “the good things” in the legislation, Mr. Bloomberg said. But he’ll be watching. Pledging to talk to every lawmaker he could, the mayor said he would tell them that if they vote for the Tiahrt Amendment, they are “voting to put guns in the hands of criminals.”

“And I’m going to try to remind every one of their constituents, particularly two years from now when they have an election,” Mr. Bloomberg said.

A permanent adoption of the Tiahrt Amendment would also strike at a key strategy of the Bloomberg administration by restricting how data is used in civil lawsuits. The city has sued 27 gun dealerships that sold guns illegally and has reach settlements with seven dealers that have agreed to a court-appointed monitor.

A spokesman for the NRA said yesterday that the lawsuits were the only reason Mr. Bloomberg wanted the data restricted by the Tiahrt Amendment. “What they’re trying to do is get this information for political reasons,” the spokesman, Andrew Arulanandam, said.

Looking to rebut attacks from the NRA, Mr. Bloomberg stressed repeatedly yesterday that his effort was not a Second Amendment issue, but a crime issue.

Yesterday’s summit in the Cannon House Office Building drew a small protest by leaders of several gun rights groups, who held a news conference across town in the New York room of the Capitol Hilton hotel to criticize Mr. Bloomberg. The president of Virginia Citizens Defense League, Philip van Cleave, announced that two gun shops — one in Blacksburg, Va., and one in Richmond — would be holding a “Bloomberg Gun Giveaweay,” in which customers who bought $100 worth of merchandise would be eligible to win a $1,000 pistol or rifle.

Asked about the promotion later, Mr. Bloomberg said the organizers were “sick people.”

At yesterday’s conference, the mayors also released poll results showing support for their cause among Americans. And they announced the creation of a bipartisan House task force, to be headed by Rep. Charles Rangel, a powerful Democrat who is the dean of the city’s congressional delegation, along with Rep. Peter King, a Republican of Long Island, Rep. John Conyers Jr., a Democrat of Michigan, and Rep. Mark Kirk, a Republican of Illinois.

Mr. Bloomberg, along with the mayor of Boston, Thomas Menino, formed the coalition last year, and it has grown to 123 mayors from 44 states.