President Obama delivered an impassioned and urgent plea in West Hartford, Conn., Monday evening for swift action, asking a state still shaken by the Newtown school massacre why there isn’t more support in Congress for gun control legislation. (The Washington Post)

New York Mayor Michael R. Bloomberg, one of the nation’s most committed and deep-pocketed gun-control proponents, is ratcheting up pressure on lawmakers by launching a new system to grade them based on their votes and statements on gun issues.

Mayors Against Illegal Guns, the nonprofit group financed by Bloomberg (I), will unveil a scoring system Tuesday to award lawmakers grades of A through F, much like the National Rifle Association, which has derived much of its power by deploying letter rankings against politicians at election time. The group’s strategists briefed The Washington Post on the plans ahead of Tuesday’s announcement.

“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,” said Mark Glaze, director of Mayors Against Illegal Guns, which is chaired by Bloomberg and is made up of more than 900 mayors from across the country.

This is Bloomberg’s latest move to establish his organization as a counterweight to the influential NRA and to create legislative momentum on Capitol Hill around President Obama’s proposals to toughen the nation’s gun laws.

Mayors Against Illegal Guns will start airing a 60-second ad on Tuesday featuring Neil Heslin, whose son, Jesse Lewis, was killed in December’s elementary school massacre in Newtown, Conn. The group is spending more than $1 million to air the ad on cable news in Washington and on television stations in 10 targeted states, officials said.

How the NRA exerts influence over Congress

The group is targeting 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 J. Toomey (R-Pa.).

The group also this week will begin airing a new television ad statewide in Pennsylvania focused on Toomey, who quietly has been negotiating on a compromise on expanding background checks with Sen. Joe Manchin III (D-W.Va.). The ad highlights Toomey’s past statements in support of the idea and urges Pennsylvanians to call his office to “demand action” when a background-check bill comes up for a vote in coming days.

The group’s new ratings system would produce a scorecard for senators and representatives that could serve as a guide for voters as well as campaign donors heading into the 2014 midterm elections and in future campaign cycles.

Glaze outlined a sophisticated algorithm that would weigh lawmakers’ votes on gun bills as well as their public statements and other actions to issue overall letter grades.

“Not every member can vote for every bill, but if not, there are other things they can and should be doing, and we will have our eye out for those as well,” Glaze said.

A good grade could reward a vulnerable lawmaker for backing tougher gun laws, the thinking goes, while a poor grade would publicly shame a politician who votes against popular ideas such as universal background checks.

Bloomberg, the billionaire mayor, has said he would tap his personal fortune to finance television ad campaigns defending lawmakers who vote for gun-control measures and attacking those who do not.

Earlier this year, Bloomberg spent more than $2 million on an ad blitz for a special primary election in an Illinois congressional district to deny the seat to NRA-backed former congresswoman Debbie Halvorson, a Democrat.

The Mayors Against Illegal Guns scoring system was developed in part because major political donors who support gun control have been seeking guidance about how to direct their contributions in upcoming election cycles, Glaze said.

Some influential Democratic benefactors, including businessman Kenneth Lerer and technology entrepreneur David Bohnett, have warned publicly that they will not write another check to Senate Democrats who do not vote to expand background checks.

“The appetite for information about where members are is very high,” Glaze said. Of the scorecard, he added: “We don’t intend that this will be a tree falling in the forest. We intend for people to have all the information they need to make sound voting and political-contribution decisions.”

The grading system is most directly a threat to Democrats from conservative states such as Arkansas, Montana and North Carolina who could feel compelled to vote against some gun measures because of the large number of gun owners in their states yet rely on donations from liberal donors in places such as California and New York to fuel their campaigns.

It is too soon to know whether Bloomberg’s new and untested scorecard will have the same impact as the NRA’s. For decades, Republicans as well as Democrats from more conservative states — including Senate Majority Leader Harry M. Reid (D-Nev.) — have campaigned on their “A” ratings from the NRA.

“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,” NRA spokesman Andrew Arulanandam said. In a dig at Bloomberg, he added, “We’ll take that over the purse of one billionaire any day of the week and twice on Sunday.”

Mayors Against Illegal Guns has been airing television ads in 10 key states for two weeks, and the results are uncertain. The group is trying to pressure lawmakers to vote for expanding background checks, but a deal in the Senate has proved elusive and many of the targeted senators have been circumspect about their positions.

“It is close to impossible to get a senator who is the subject of an ad to tell you whether it helps or it hurts,” Glaze said. “I think the proof will be in what the Senate ends up producing.”

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