(CNN) - The National Rifle Association is famous for its scorecard that gives lawmakers a letter grade based on their votes against gun restrictions.
Now, Mayors Against Illegal Guns–a group battling the NRA in the recent gun control debate–is using their opponent's tactic against them, creating their own grading system for lawmakers, some of them facing re-election next year.
Mark Glaze, director of the mayors group, which is co-chaired by New York Mayor Michael Bloomberg and includes more than 900 mayors, confirmed they're launching the grading system. The story was first reported by The Washington Post.
"We're asked many times daily where people's elected representatives are on gun laws, and we intend to tell them, in detail," Glaze told CNN.
At a press conference later Tuesday, Bloomberg said grades will be based on both past and future votes.
"I hope it will make them understand that the NRA is not the only one that's scoring them. And that other people are looking at what they do," he said. "You know you vote for people based on what they did in the past. If you vote on the basis of what they say they're gonna do in the future you'll be sorely disappointed and it'll be a very inaccurate science."
Bloomberg also sounded open to grading state lawmakers, as well.
The group will spend $1 million on cable television ads in the Washington area and in 10 states.
According to the ratings system, the more often a lawmaker supports tougher gun legislation, the higher score they will receive. With the NRA, the scoring works in the reverse.
The new campaign comes as the Senate is expected to begin debate over proposed gun legislation as early this week. The most likely provision to pass includes an expansion of the background check system.
Bipartisan negotiations over the measure, however, may fall short of the proposal backed by President Barack Obama and many Democrats. The provision would expand the system to cover transactions at gun shows and over the Internet, but would not require them for any other private sales, CNN's Dana Bash reported.
Many of the 12 targeted senators in the ad campaign are Republicans, with nearly half being Democrats from conservative-leaning states. The list of lawmakers includes:
Saxby Chambliss, R-Georgia (A+ with NRA)
Dan Coats, R-Indiana (C+ with NRA)
Joe Donnelly, D-Indiana (A with NRA)
Jeff Flake, R-Arizona (A with NRA)
Kay Hagen, D-North Carolina (F with NRA)
Heidi Heitkamp, D-North Dakota (AQ* with NRA. As a freshman senator, she does not have a voting record yet)
Dean Heller, R-Nevada (A with NRA)
Johnny Isakson, R-Georgia (A with NRA)
Mary Landrieu, D-Louisiana (C with NRA)
Rob Portman, R-Ohio (A with NRA)
Mark Pryor, D-Arkansas (C- with NRA)
Pat Toomey, R-Pennsylvania (A with NRA)
Hagan, Landrieu, and Pryor each face re-election in 2014. Chambliss, meanwhile, has announced he's not seeking another term.
- CNN's Todd Sperry, Jason Kessler, Paul Steinhauser and Ashley Killough contributed to this report.
This week, CNN TV and CNN.com will take an in-depth look at “Guns Under Fire: A CNN Special Report On Background Checks.” On Tuesday night at 8 p.m., AC360 will debut an exclusive interview with former Rep. Gabrielle Giffords, who was shot in the head two years ago in Arizona. On Wednesday, the network will look at gun control and background checks as Congress is expected to tackle the issue head-on in the coming days. Watch CNN TV and follow online at CNN.com or via CNN's apps for iPhone, iPad and Android.