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)

As the Senate prepares to begin debate next week on the biggest gun-control bill in nearly two decades, the gun rights lobby and its Senate allies are working on a series of amendments that could have the opposite effect — loosening many of the restrictions that exist in current law.

Most worrisome to those who advocate new gun limits is an expected amendment that would achieve one of the National Rifle Association’s biggest goals: a “national reciprocity” arrangement, in which a gun owner who receives a permit to carry a concealed weapon in any one state would then be allowed to do that anywhere in the country. Other pro-gun proposals would make it easier for dealers to sell their merchandise between states or let certain people who had been treated for mental illness regain the right to buy weapons.

The freewheeling Senate chamber has always been a place where legislation can take an abrupt detour. But rarely has there been so much potential for unpredictable turns as on gun legislation, where allegiances break down not along party lines, but along regional ones and an urban-rural divide.

Among the Democrats who have voted in recent years for bills that would expand gun rights are Majority Leader Harry M. Reid (Nev.) and several rural-state senators facing potentially difficult reelection bids in 2014: Max Baucus (Mont.), Mark Begich (Alaska), Kay Hagan (N.C.), Mary Landrieu (La.) and Mark Pryor (Ark.).

President Obama has been working behind the scenes to round up resistant Democrats. He telephoned Begich on Tuesday afternoon and spent 15 or 20 minutes discussing the upcoming gun votes, Begich spokeswoman Rachel Barinbaum said.

The Senate debate is scheduled to get underway Tuesday. At this point, it is not assured that any legislation will pass in the end. While the Senate voted 68 to 31 to allow the debate to begin, members in either party could still find ways to stall or block every amendment or the final passage of the entire bill.

In practice, that means anything controversial would need 60 votes, not a simple majority of 51, to reach the threshold needed to avoid a filibuster. There are 53 Democrats in the Senate, plus two independents who usually vote with them.

With a significant number of Democrats inclined to vote their home-state preferences and side with Republicans in support of expanded gun rights, advocates of firearms limits have had to accept the reality that any legislation that passes the Senate would fall far short of the more restrictive proposals put forward by Obama in January.

As a result, amendments to impose a ban on military-style assault weapons and high-capacity magazines are thought to have little chance.

Meanwhile, gun-control advocates face the possibility that the chamber might pass a bill that undercuts the current law in many respects.

Republicans said the prospect of having many amendments, an assurance they got from Reid, was one of the reasons that so many of them voted in favor of allowing the bill to proceed.

For instance, the breakthrough bipartisan agreement by Sens. Joe Manchin III (D-W.Va.) and Patrick J. Toomey (R-Pa.) to expand background checks on many firearms purchases, which will be the first item upon which the Senate votes, also includes the language allowing gun dealers to more easily market and sell their products between states.

Meanwhile, Sens. Charles E. Grassley (R-Iowa) and Ted Cruz (R-Tex.) were putting together a broad alternative gun bill that would include a previous proposal offered by Sen. Lindsey O. Graham (R-S.C.) designed to streamline mental health reporting to the national background check system. Gun-control advocates say the mental health provision would loosen barriers for those with serious mental illnesses to acquire firearms.

Graham rejects that criticism, saying his legislation would clarify and improve the law. Among its features is one that would restore gun ownership rights to people deemed to have recovered enough to no longer need treatment for mental illness.

Few proposals are likely to spark as much controversy as the one regarding concealed weapons. The NRA has lobbied successfully in dozens of states for concealed-carry permit programs, and winning “national reciprocity” has been a long-held goal for the group.

“Congress should recognize that the right to self-defense does not end at state lines,” NRA lobbyist Chris W. Cox said in a statement issued last month, when the proposal was introduced in the Senate.

Sen. Charles E. Schumer (D-N.Y.) described the measure this week as “the most pernicious” proposal under consideration.

“Somebody could come from Wyoming to the big cities of New York or New Haven or Bridgeport and carry a concealed weapon, which is so against our way of life, and the needs here in New York,” Schumer said.

The reciprocity proposal was last put to a vote by the Senate in 2009 and received 58 votes — just two short of the necessary 60.

Gun-control advocates are studying that earlier vote tally in hopes of identifying Democrats they can persuade to switch sides and oppose the reciprocity provision.

Among the possibilities, gun-control advocates say, are the two Colorado senators, Mark Udall and Michael F. Bennet. Since that earlier vote, Colorado experienced the 2012 mass shooting at an Aurora movie theater. State lawmakers have also passed legislation requiring background checks for private and online gun sales and banning ammunition magazines that hold more than 15 rounds.

Some Senate staff members were also working on possible alternatives that might win over red-state Democrats, including one that would allow reciprocity to apply only in cases where the state that issues a concealed-weapons permit meets a set of minimum standards for doing so.

If the reciprocity proposal passes in the form desired by gun rights advocates, those on both sides think it could jeopardize the entire gun-control bill.

It would make the bill unacceptable “because it so significantly overrides and undermines the states’ ability to set public safety laws,” said Arkadi Gerney, a senior fellow at the liberal Center for American Progress.

Added Richard Feldman, a former NRA lobbyist who now heads the Independent Firearm Owners Association: “Part of me likes [the reciprocity provision], but from a legislative strategy, it becomes a killer amendment. The same votes that would be for background checks, lots of them would vote no on the final bill.”

Sen. Jeff Flake (R-Ariz.), who voted this week to debate the gun bill, said he voted to proceed because Reid promised an open amendment process.

“Most of us in the Republican caucus, what we’re looking for regarding motions to proceed are fair treatment,” Flake said. “That hasn’t been the case in the past couple of years and, if it is, you’ll see a lot more I think support for Republicans to proceed.”

Tom Hamburger contributed to this report.

Discuss this topic and other political issues in the politics discussion forums.