“It gets really, really frustrating in areas like this where a mayor does not have the control,” said Mayor Steve Adler of Austin, Texas, a Democrat in a Republican-controlled state. He added that it was hard to tell a constituent demanding new laws that “‘I agree with you and I wish I could change that but I have no control.’ That’s an insufficient answer.”

Mayor Kim Norton of Rochester, Minn., who unsuccessfully pushed gun control bills in her former role as a Democratic state lawmaker, said she wished the State Legislature, where Republicans control one chamber, would at least allow her to enact municipal gun restrictions, like banning weapons at the library or creating a multiweek waiting period to buy a gun. There is no indication that it will. Ms. Norton is no longer affiliated with a political party.

Republicans opposed to more gun control often point to places like Illinois, where guns are tightly regulated but homicides remain pervasive, as evidence that new restrictions will not meaningfully reduce violence. Many of them have called instead for investing in mental health programs, increasing funding for law enforcement and bolstering security at schools. Democrats say they are missing the point.

“We’re not having the same conversation,” said Mayor Satya Rhodes-Conway of Madison, Wis., who wants stricter gun laws. “Half of the conversation is about, ‘How can we keep letting people die unnecessarily?’ and half of the conversation is, ‘I need my gun, it’s my constitutional right.’ We’re not even at the same table talking about it.”

Ms. Rhodes-Conway was among several mayors who described constantly bracing for a late-night alert about a mass shooting in their city.

For Mayor Jim Kenney of Philadelphia, that came Saturday evening in Reno during a routine phone check-in with an aide. More than a dozen people had been shot at a popular weekend gathering spot, he was told, and three of them had died.

Mr. Kenney, a Democrat who was scheduled to leave the Nevada conference on Sunday night, said that he looked into catching an earlier flight or even driving to the airport in San Francisco but that there were no good options to make it home sooner. “It’s like, ‘What? What?’” Mr. Kenney said of his initial reaction to the shooting. “And I can’t get out.”