The coalition of more than 900 mayors across the country was expected to announce a scoring system Tuesday, with grades of A through F, much like those of the National Rifle Association, Director Mark Glaze told The Washington Post.
"For decades, the NRA has done an admirable job of tracking to minute detail how members of Congress stand on gun bills. We've simply decided to do the same," Glaze said.
NRA spokesman Andrew Arulanandam told the Post the Mayors Against Illegal Guns scoring system would likely not have the same effect as the NRA's.
"The reason NRA scorecards are effective is that they have the weight of approximately 5 million dues-paying members and tens of millions of other supporters behind them," he told the Post.
"We'll take that over the purse of one billionaire any day of the week and twice on Sunday," he added.
Mayors Against Illegal Guns, largely financed by billionaire Bloomberg, says it has more than 1.5 million grassroots supporters, which it says makes it the nation's largest gun violence prevention advocacy group.
The scoring system is part of Bloomberg's move to establish Mayors Against Illegal Guns as a powerful counterbalance to the NRA, the Post said.
The group also was expected to start airing a 60-second commercial Tuesday featuring Neil Heslin, the father of 6-year-old Jesse Lewis, one of the 20 schoolchildren killed in the Dec. 14 mass murder at Sandy Hook Elementary School in Newtown, Conn.
The group is spending more than $1 million to air the spot in areas where it can target Sens. Saxby Chambliss, R-Ga., Daniel Coats, R-Ind., Joe Donnelly, D-Ind., Jeff Flake, R-Ariz., Kay Hagan, D-N.C., Heidi Heitkamp, D-N.D., Dean Heller, R-Nev., Johnny Isakson, R-Ga., Mary Landrieu, D-La., Rob Portman, R-Ohio, Mark Pryor, D-Ark., and Patrick Toomey, R-Pa., officials told the Post.
The group also plans to air a new TV spot in Pennsylvania focused on Toomey, who is negotiating a bipartisan deal to expand gun-purchase background checks with Sen. Joe Manchin, D-W.Va., the officials said.
The ad highlights previous Toomey statements in favor of the idea and urges Pennsylvanians to phone his office to "demand action" when the background-check bill comes up for a vote, the Post said.
The Mayors Against Illegal Guns scoring system was developed partly because big political donors who support gun control wanted guidance about how to direct their contributions in upcoming election cycles, Glaze told the newspaper.
The next big election cycle is the 2014 midterm elections, when all 435 House members are up for election and a third of the 100 Senate seats are up for grabs.