The National Rifle Association has introduced a new letter grade rating, “Fx”, for candidates endorsed by its political rival, America’s largest gun control group.

Everytown For Gun Safety, the group targeted by the new NRA rating, announced on Monday that it would be spending $2m in Florida before the midterm elections to support the candidates it has endorsed.

The new Fx rating is the brainchild of Marion Hammer, the powerful 79-year-old NRA lobbyist who has dominated Florida politics for years, according to a source familiar with the ratings.

Midterm elections in Florida, nicknamed the “Gunshine State” for its loose gun laws and embrace of pro-gun politicians, have become a key battleground in the fight over gun control in America.

Among the Florida recipients of the NRA’s new Fx grade are Senator Bill Nelson, Congressman Ted Deutch and Andrew Gillum, a Democratic candidate for governor of Florida. All three Democratic candidates have championed the gun control activism of the teenage survivors of the 14 February school shooting in Parkland, Florida, which left 17 people dead.

Students are released from lockdown on 14 February at Marjory Stoneman Douglas high school in Parkland, Florida. Photograph: Sun Sentinel/TNS/Sipa USA/REX/Shutterstock

Gun control advocates are hailing the Fx grade as a sign of the growing influence of the movement for stricter gun control laws, which Republican politicians have successfully blocked at the federal level for the past 25 years, despite a series of increasingly frequent and deadly high-profile mass shootings.

The NRA has long graded state and national candidates from “A” to “F” based on their record of opposition to new gun control laws. Legislators earned A+ grades from the NRA with “an excellent voting record on all critical NRA issues”. Politicians who failed to support some pro-gun laws, or who “supported some restrictive legislation”, got Bs or Cs. The old F-rating from the NRA was given to a politician who actively led or sponsored gun control legislation, making them a “consistent anti-gun candidate” and “true enemy of gun owners’ rights”, according to the group’s ranking system.

The new NRA grade, Fx, targets candidates who have been endorsed by Everytown for Gun Safety and its affiliate Moms Demand Action for Gun Sense in America, advocacy groups supported by Michael Bloomberg, the billionaire former mayor of New York City.

The NRA has introduced a new letter grade, ‘Fx’, is applied to Florida candidates who are ‘consistent anti-gun candidates’. Photograph: NRAPVF.ORG

The Fx rating is only being applied to candidates in Florida, according to Jennifer Baker, a spokeswoman for the NRA’s Institute for Legislative Action. The rating marks a candidate “who requested and received the endorsement of Bloomberg’s anti-gun groups”, according to the NRA’s web site.

While the Fx rating is new, the NRA “has always factored the endorsements of gun ban organizations into our ratings”, Baker wrote in an email.

John Feinblatt, Everytown’s president, greeted the new NRA ranking as a victory.

“If there was any doubt that Everytown was the counterweight to the NRA, this proves it once and for all,” Feinblatt told the Guardian.

Everytown and other gun violence prevention organizations have been working for nearly six years, since the 2012 Sandy Hook Elementary school shooting, to organize a gun control movement powerful enough to convince congressional Republicans that they can no longer block gun control laws and stay in office. So far this year, Everytown has endorsed more than 100 state and federal candidates in the 2018 midterm elections.

The gun control group was founded in 2014, after Congress failed to pass any gun control legislation after the Sandy Hook elementary school shooting in 2012, which left 26 people dead. It has no official memberships, but it claims more than 350,000 donors nationwide, as well as roughly 5 million supporters who have signed up to volunteer with or receive emails from different branches of the organization.

The NRA publicizes its letter rankings, as well as its endorsements, to its millions of members nationwide, urging them to support or oppose candidates based on the single issue of gun rights. While opponents of the gun rights group often focus on the millions it donates to political candidates, researchers say the NRA’s real power has come from its ability to mobilize voters and swing Republican primary elections in favor of pro-gun candidates.

Florida had long been a kind of test lab for new pro-gun legislation. The state’s 2005 Stand Your Ground law, which encourages citizens to use deadly force to protect themselves in public, was later passed in more than two dozen other states.

Florida’s current governor, Rick Scott, has been downgraded from an A+ to a C since he signed into law modest gun control compromises after the Parkland shooting. Photograph: NRAPVF.ORG

Hammer, the first female president of the NRA and its longtime Florida lobbyist, was a key force in shaping the state’s pro-gun legislative environment. In the wake of the Parkland shooting, some of the state’s Republican lawmakers defied Hammer and passed a slate of modest gun control laws. Hammer publicly vowed that they would be held accountable for their “selfish betrayal” of gun rights. The lobbyist did not immediately respond to a request for comment on the NRA’s new Florida Fx ratings.

Gun control advocates have pointed to the passage of new state-level gun control laws in Florida as evidence that the political landscape in the United States is changing, and becoming more favorable to politicians who support additional moderate gun control.

Florida’s current governor, Rick Scott, who once had an A+ rating from the NRA, has now been downgraded to a C. Scott signed into law the modest gun control compromises after the Parkland shooting, which included raising the minimum age to buy a firearm from 18 to 21. The perpetrator of the Parkland high school shooting, an 18-year-old former student, had been able to legally buy the military-style semi-automatic rifle he used in the shooting.