(CNSNews.com) - All three presidential hopefuls have said they want to "close the gun show loophole," and Mayors Against Illegal Guns is holding them to their word.

The gun-control group on Monday unveiled a new television advertisement, in which Democratic Sens. Hillary Clinton and Barack Obama and Republican Sen. John McCain are heard commenting on the so-called "loophole." Clips of their comments were taken from various speeches.

Featured in the ad are Mayor John Peyton of Jacksonville, a McCain supporter; Mayor Sheila Dixon of Baltimore, an Obama supporter; Mayor Thomas Menino of Boston, a Clinton supporter; and Mayor Michael Bloomberg of New York, who describes himself as an undecided voter.

The ad will run nationwide on cable, as well as network television, Mayors Against Illegal Guns said.

The group held its national summit in Washington on Monday -- a gathering intended to "shine a spotlight on the problem of illegal guns flowing into America's cities," the mayors said in a news release.

"You can walk into a gun show, having a criminal record a mile long, and just walk up to a counter and buy 20 guns and walk out the door with no checks and no federal restrictions whatsoever," Bloomberg said. "It is just craziness."

In fact, federally licensed firearms dealers who sell at gun shows must abide by all local, state and federal laws governing gun sales. That includes conducting the required background checks.

The so-called "loophole" involves private sales among individuals. People wishing to sell their own property can make private gun sales -- at gun shows or at any other location -- as long as they don't have any "reasonable cause" to believe the buyer is prohibited from possessing firearms under federal law.

Critics describe these private sales -- which can take place anywhere -- as the "gun show loophole" because gun shows draw willing buyers and willing sellers together and private sales are part of such events.

'Responsible retail partnership'

During Monday's summit, Mayors Against Illegal Guns also announced a "responsible firearms retail partnership" with Wal-Mart, which is a big seller of rifles and shotguns.

Wal-Mart has agreed to videotape gun-sale transactions in its stores -- and keep the videotapes -- in case police want to see them later as part of an investigation.

Wal-Mart also agreed to expand background checks of employees who handle guns; and it will create a system to track when a gun sold at Wal-Mart is later used in a crime. The gun buyer would then be barred from purchasing any more firearms at Wal-Mart stores.

The National Shooting Sports Foundation (NSSF) -- the firearms industry's trade association -- said the Wal-Mart deal is nothing but a publicity stunt.

The partnership's name -- the "responsible firearms retail partnership" -- "gives the false impression that federally licensed firearms dealers are somehow irresponsible," said NSSF Senior Vice President and General Counsel Lawrence G. Keane.

Moreover, NSSF noted that it has never opposed background checks on firearm retailer employees. In fact, NSSF said it would consider supporting legislation to allow gun dealers to conduct background checks on prospective employees through the National Instant Criminal Background Check System, something that is currently prohibited by law.

According to Keane, Mayor Bloomberg has rebuffed past requests to educate him about the firearm industry's cooperative relations with law enforcement in helping to reduce criminal access to firearms.

Through a program called "Don't Lie for the Other Guy," the NSSF helps the ATF to better educate America's firearms retailers on how to detect would-be straw purchasers and to raise public awareness that it is a serious crime to buy a firearm for a prohibited person.