The Senate agreed Thursday to move forward on gun control legislation but in addition to expanding background checks the new law would accommodate a long-standing priority of the National Rifle Association. The Post’s Karen Tumulty explains. (Christina Lee/The Washington Post)

Pennsylvania, Georgia and Virginia have long been bastions of gun-rights supporters, with vast rural areas and strong hunting traditions. But in recent days, lawmakers from those states have demonstrated a new willingness to back stricter firearms regulations, setting the stage for what could be the first major gun-control legislation to pass Congress in two decades.

The shift underscores a new reality of gun politics in America: The rapid growth of suburbs in historically gun-friendly states is forcing politicians to cater to the more centrist and pragmatic views of voters in subdivisions and cul-de-sacs as well as to constituents in shrinking rural hamlets where gun ownership is more of a way of life.

In Pennsylvania, for example, all 14 Republicans representing the state in Congress are solidly pro-gun and have “A” ratings from the National Rifle Association. Yet the state’s conservative Republican senator is the co-architect of a new compromise to expand background checks for firearms purchases, and a handful of GOP House members from the state’s suburban areas are poised to back the measure.

The central role played in recent days by Sen. Patrick J. Toomey (R-Pa.) has brought attention to how the Philadelphia and Pittsburgh suburbs have become the state’s political centers of gravity. Similar shifts can be seen in other states with fast-growing suburbs, including Georgia, Virginia, Arizona and Colorado.

The phenomenon will be on wider display next week when the debate in Congress begins. The gun legislation cleared its first major hurdle Thursday when the Senate voted 68 to 31 to proceed with debate. Sixteen Republicans joined 52 Democrats in supporting initial debate, overcoming the threat of a filibuster by Minority Leader Mitch McConnell (Ky.) and other Republicans.

Sen. Timothy M. Kaine (D-Va.) estimated that his constituency is 65 percent suburban — and those voters, he said, are looking for “very practical, middle-ground solutions” from their elected officials.

“There is a respect for the Second Amendment,” Kaine said. “But we definitely understand, too, that there are balances.” Suburbanites, he said, believe that gun rights should be “tempered by social responsibility.”

Unlike every other debate that has unfolded recently in a bitterly divided Washington, the gun debate is much more about geography than party. The dividing lines are not between Democrats and Republicans, but between rural lawmakers and those who must cater to urban and suburban constituencies.

This explains why Mayors Against Illegal Guns, the gun-control group financed by New York Mayor Michael R. Bloomberg (I), has been airing television ads in Atlanta, Philadelphia, Phoenix, Las Vegas and such Ohio metropolitan areas as Cleveland, Columbus and Cincinnati, sensing opportunities to sway Republican senators.

The rural-suburban divide was evident in the role played in recent weeks by Sen. Tom Coburn (R-Okla.), a gun-rights backer. Democrats looking for a Republican partner to broker a bipartisan deal on background checks first turned to Coburn, a staunch conservative with a penchant to reach for compromise. Coburn spent weeks negotiating with Sens. Joe Manchin III (D-W.Va.) and Charles E. Schumer (D-N.Y.), but the talks stalled as the Oklahoman faced intense lobbying pressure from gun advocates in his heavily rural state.

Democrats then turned to Toomey, whose political survival depends largely on winning over suburban voters in the Philadelphia and Pittsburgh areas.

The geographic divisions are playing out among Democrats, as well. Two of the Senate votes on Thursday against proceeding with debate on the gun-control legislation came from Sens. Mark Begich of Alaska and Mark Pryor of Arkansas, Democrats whose home states remain largely rural.

In the end, the compromise plan on background checks drafted by Toomey, Manchin and Schumer fits the centrist nature of the suburbs, where polls show voters to be mixed on gun-control issues. The plan would expand background-check requirements on gun purchases. But in not covering many sales between private individuals, the measure stops far short of the stricter controls sought by President Obama and gun-control advocates.

“On the one hand, this compromise was enormously bold and took real political courage,” said Matt Bennett, a gun-control proponent and senior vice president at Third Way, a centrist think tank. “But on the other hand, it was very sensible and smart politics because it really meets the moment that we’re in, which calls for that kind of flexibility and compromise.”

Even so, the the demographic shifts are creating complicated political equations for many lawmakers, particularly senators, who must balance the need to represent their states’ suburbs while also factoring in a pro-gun voter base that might be smaller in number but more energized. The NRA, with about 5 million members, remains a potent grass-roots force, even in states with fast-growing suburbs.

“An intense minority trumps an apathetic majority,” said Whit Ayres, a longtime Republican pollster. “On no issue is that more true than on gun issues.”

Consider Georgia, where a growing number of voters live in the suburbs sprouting up around Atlanta.

Sen. Saxby Chambliss (R-Ga.) acknowledged that the constituents calling his offices in his once overwhelmingly pro-gun state are split.

“I’ve probably heard from as many pro-Second Amendment folks as well as the same number of folks who’d like some sort of measures put in place to control,” Chambliss said, noting that he has not decided whether to vote for the background-checks compromise.

Toomey said reaction in Pennsylvania to his Wednesday announcement of the compromise has been mixed. He said he plans to spend the next few weeks explaining the intricacies of his proposal to voters back home, as well as to his colleagues in Washington.

“The more people learn about what this bill actually does — how it does it, how reasonable it is, the fact that it doesn’t undermine any law-abiding citizen’s Second Amendment rights — I think support will grow,” Toomey said.

The most likely Republicans to lead a bipartisan push on gun legislation, especially in the House, appear to be those representing suburban districts.

The three-county ring of suburbs around Philadelphia has long been one of the central battlegrounds for control of the House. Once solid turf for centrist Republicans, voters there have backed Democrats for president in the past six elections — but have split their votes in congressional races.

To win back the House in 2014, Democrats have targeted those seats, as well as dozens of other similarly suburban districts, with a recruiting effort focused on finding non-ideological “problem solvers.”

One such Republican incumbent being targeted by Democrats is Rep. Patrick Meehan (Pa.), a former prosecutor who easily won reelection last year in the Philadelphia suburbs. In February, he stood alongside Rep. Elijah E. Cummings (D-Md.) at a news conference backing a bipartisan proposal to make gun trafficking a federal crime and penalize “straw purchases” in which guns end up with other owners.

“People at home are sort of looking for Congress to be a little more proactive in dealing with the nation’s issues and challenges,” Meehan said. “So to the extent that we aren’t responsive to issues of the moment, there’s a growing sense of frustration.”

Ed O’Keefe and Aaron C. Davis contributed to this report.

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