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A coalition of U.S. mayors released plans this morning to take members of Congress to task for what the group perceives as votes against gun control.

Mayors Against Illegal Guns, the group co-chaired by New York City Mayor Michael Bloomberg, will give lawmakers a letter grade and issue a scorecard based on their gun-policy record, group spokesman John McCarthy said.

The system is intended to counterbalance the National Rifle Association's scoring political candidates on their friendliness toward gun owners.

The NRA's letter ratings have typically translated to monetary support for candidates who score favorably, and the organization has used the process as a tool of influence on hot-button political debates, like the confirmation of Supreme Court Justice Elena Kagan.

Politicians tout their NRA "A" rating as testimony to their commitment to protecting Americans' Second Amendment rights.

But Bloomberg's grades will mostly measure the opposite, taking aim at some of the NRA's A-listers like Sen. Rob Portman, R-Ohio, and Sen. Joe Donnelly, D-Ind., for any votes they make against gun control, according to The Washington Post.

The grades will take into account stances members of Congress have taken on past legislation, as well as upcoming votes.

Bloomberg said he wants Congress to understand that "the NRA is not the only one scoring them."

"For decades the NRA has been the only voice talking about guns," he told reporters today. "They're up in arms someone's telling a different story."

Mayors Against Illegal Guns includes about 900 mayors from across the country, according to Bloomberg. Members of the organization have pushed for universal background checks and an assault-weapons ban since days after the December shooting at Sandy Hook Elementary School in Newtown, Conn.

In the weeks after Sandy Hook, lawmakers seemed to have momentum toward passing restrictions on gun ownership and sales, but fervor for gun control in Washington has cooled in recent weeks, despite President Obama's multi-state push to promote his package of tougher laws.

More than a dozen Republicans, led by Sen. Rand Paul of Kentucky, have said they will block a vote on any gun control legislation that comes up in the Senate.

Obama, speaking before families of Sandy Hook students Monday, condemned such efforts, calling them "political stunts."

"They're saying they'll do everything they can to even prevent any votes on these provisions," Obama told the crowd in Connecticut."They're saying your opinion doesn't matter and that's not right. That is not right."

Senate Majority Leader Harry Reid, D-Nev., urged Republicans to put aside their filibuster plans for the sake of Newtown victims.

"It is impossible to prevent every senseless tragedy, but we owe it to our children to at least try," Reid said. "I hope Republicans will stop trying to shut down debate, start engaging on the tough issues we were sent to Washington to tackle."

ABC News' Aaron Katersky contributed to this report.