Having already won their most important priority — Supreme Court recognition of an individual constitutional right to bear arms — gun rights groups are increasingly fighting on terrain where they have less support, including pushing bills at the state and local level to carry concealed weapons in virtually any public setting. The N.R.A. continues to fight aggressively to dismantle existing law enforcement gun databases and to defeat efforts to apply background checks to more gun purchasers, measures that typically have solid public support.

In the post-Citizens United world, where checks from a handful of billionaires can rival the fund-raising of an entire presidential campaign, the N.R.A.’s treasury gives it less clout than before. The group’s $17 million in outside spending in 2012 was a small fraction of the total spent by the big outside groups. Moreover, some opponents believe the N.R.A.’s ever-tighter relationships with Republican officials and an electorate that evermore comprises suburban and urban voters who are female and nonwhite, give it less leverage over Democrats, even in red states.

On Monday, two pro-gun-rights Democrats, Senators Joe Manchin of West Virginia and Mark Warner of Virginia, said they would consider supporting new measures to limit guns. Both have “A” ratings from the N.R.A.

But any such measures would face an uphill battle. In 2009, the N.R.A. failed to muster enough votes in the Senate to pass an amendment allowing anyone granted a concealed-weapons permit in any state to carry their gun in any other state. Gun control groups hailed it as the N.R.A.’s first defeat in a floor vote in years — but 58 senators voted for the amendment.

Over the years the N.R.A. has perfected its strategy for responding to mass shootings: Lie low at first, then slow-roll any legislative push for a response.

After the shootings at Columbine High School in 1999, for example, an effort to close the so-called gun-show loophole, requiring unlicensed dealers at gun shows to run background checks, ultimately died in conference after being stalled for months.

After the massacre at Virginia Tech in 2007, Congress did manage to pass a modest measure that was designed to provide money to states to improve the federal background check system. But the N.R.A. secured a broad concession in the legislation, which pushed states to allow people with histories of mental illness to petition to have their gun rights restored.