"It is my hope that President Obama and Congress will work together right away to reform our gun background check system so that all records of dangerous people are in the system and all gun buyers will have to pass a thorough background check, and that this be done in a way that does not infringe on Second Amendment rights, so that responsible citizens may exercise their right to bear arms," Hernandez said in a press release.

The event is hosted by Mayors Against Illegal Guns, the coalition of 550 mayors that was co-founded by Mayor Bloomberg, who has pushed aggressively for more thorough background checks in the wake of the Arizona shooting. Yesterday, he hosted Martin Luther King III and dozens of gun violence victims at City Hall, followed by an appearance on the CBS Evening News with Katie Couric.

In an open letter published in today's Washington Post, Mayors Against Illegal Guns reiterated its central goals: ensuring background checks for every gun purchase and using data about mental health records and drug use to bar some people from buying guns. Some states have not submitted mental health data into a national database, diminishing the effectiveness of checks mandated by the Brady Bill.

Hernandez has emerged as a spokesperson for the cause, joining gun control advocates in questioning whether alleged shooter Jared Loughner should have been able to obtain a gun.

“It is a terrible piece of legislation,” said Bloomberg, and the only way it would pass is if senators who support it are “holding your nose and praying nobody sees you doing it.”

Also on the call were the mayors from Philadelphia and Milwaukee, who are both Democrats, and the mayor of Jacksonville, a Republican.

Bloomberg was asked what message he’d have for senators who are on the fence over this issue, and for Senator Harry Reid, the Democratic majority leader who, according to one reporter on the line, may support the bill.

Bloomberg said he hadn’t spoken to Reid about it and explained, “What you say is you’re not going to be able to hide what your vote is and this is just so anti-police, anti-public safety legislation, you will be held accountable by the voters.”

The group ran a full-page ad in USA Today.

Bloomberg said that there had been no public hearings on the amendment, and said it was being attached to a military spending bill, providing senators an excuse to vote for it. Bloomberg said if the amendment stood alone, it would not pass, and that “the hypocrisy of tying it to the safety of our young men and women who are overseas is just so outrageous.”