WASHINGTON — Mayor Bloomberg is gearing up for the third annual Washington summit of his coalition against illegal guns, a group that now includes more than 300 mayors nationwide along with county executives and state legislators.

The coalition is looking to expand its issue agenda and will push for enhancing the federal background check system to keep criminals and the mentally ill from buying guns. The meeting on Monday will come two days before the first anniversary of the massacre at Virginia Tech, where a mentally unstable student shot 32 people and killed himself in the deadliest school shooting in American history.

Other topics that are likely to come up at the summit are a number of proposals percolating on the state and local level, including a push to require “microstamping” tracing technology on handguns and stiffen requirements for dealers to report lost and stolen firearms.

The conference will also feature a presentation of the New York Police Department’s first-in-the-nation gun offender registry, which was implemented a year ago and allows law enforcement to track violent criminals who have been convicted of a weapons charge.

When Mr. Bloomberg launched the coalition in 2006, it was seen as a vehicle for raising his national profile. The mayor’s travels across the country, in which he railed against the scourge of illegal guns and government inaction on a range of other issues, helped fuel speculation that he would seek the presidency.

He quickly drew the ire of the National Rifle Association and other gun rights advocates, which in turn helped bring more attention to the issue. But now that Mr. Bloomberg has ruled out a White House run, will it be even harder for him to make himself heard on an issue that has faded from the national spotlight?

“That’s not a concern,” a City Hall spokesman, Jason Post, said, noting that the coalition’s membership has steadily expanded and now numbers more than 300 mayors from both parties, along with a new branch of county executives announced this week.

Yet there are signs that Mr. Bloomberg’s reign as a chief lightning rod for the gun lobby may be winding down. An alliance of gun rights groups reacted to last year’s summit by holding a competing event across town, and organizers later held a highly publicized “Bloomberg Gun Giveaway” in Virginia to protest the Bloomberg administration’s lawsuits against gun dealers.

This year, the response has been little more than eye-rolling. “Bloomberg crying about how awful guns are is basically like the rooster crowing at dawn,” a legislative adviser for Gun Owners of America, Michael Hammond, said. “It’s background noise.”

The coalition suffered a setback last year with the failure of its top federal legislative priority: repealing an amendment annually attached to an appropriation bill that restricts how law enforcement can use gun tracing data. Mr. Bloomberg and his aides lobbied intensely, but their bill did not make it out of committee.

Mr. Bloomberg’s hard-edged lobbying tactics alienated some lawmakers, but gun control advocates have stood by him. “I think we’re all on the same page,” the executive director of New Yorkers Against Gun Violence, Jackie Hilly, said. “When you’re pushing controversial measures, you have to stand up and say what you believe.”

She dismissed concerns that Mr. Bloomberg’s scaled-down political ambitions would diminish his influence on the issue. “Mayor Bloomberg is a national leader, whether or not he’s running for president,” Ms. Hilly said.

In advance of Monday’s summit, the coalition released the results of a poll finding strong support for its agenda. According to the survey, conducted by Republican and Democratic polling firms, more than 85% of Americans support requirements for reporting lost or stolen guns; prohibiting sex offenders from purchasing firearms, and requiring dealers to perform background checks on employees. Solid majorities also support requiring criminal background checks at gun shows and measures to prohibit people on terrorist watch lists from buying guns.

“We should stop listening to the pundits who say that fighting illegal guns are an unpopular issue — people support reasonable restrictions that will make us safer,” Mr. Bloomberg said in a statement accompanying the release of the poll.