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)

Moments after clearing the first procedural hurdle, Democrats and gun control groups began readying themselves for a potentially more difficult fight: Weeks of Senate debate defending their carefully crafted legislation against possible amendments — particularly a plan to allow gun owners to carry concealed weapons from one state to another — that would kill the bill’s underlying goal.

If previous Senate votes are any guide, the concept of “national reciprocity” enjoys enough support in the Senate to pass.

The proposal won bipartisan approval in the GOP-controlled House last year with a margin of 272 to 154. And the National Rifle Association, which has successfully lobbied dozens of state legislatures over the years to allow concealed weapons permits, is expected to push for the provision’s inclusion in the new legislation.

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

Sen. Charles E. Schumer (D-N.Y.) described the measure this week as “the most pernicious” proposal under consideration and said approving the plan would mean that “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.”

Senate Democrats are confident that they’ll be able to keep the reciprocity proposal from passing, with senior aides believing that some Democrats who previously voted for it will flip their votes. Last year, Sen. Dianne Feinstein (D-Calif.) blocked the proposal in the Senate, citing what she said were potential dangers to domestic violence victims and police officers.

But two new key players in the gun debate, Sens. Joe Manchin III (D-W.Va.) and Patrick J. Toomey (R-Pa.), both with long records of backing expanded gun rights, said this week that their bipartisan proposal to expand the gun background check system amounted to the first step towards national reciprocity. And the presence of a number of pro-gun Democrats could mean the votes are there to make it happen.

“I hope we get there,” Toomey told reporters.

The bill filed Thursday by Manchin and Toomey includes some new provisions that gun rights advocates like — including one that allows gun sellers to more easily transport their merchandise between states. However, the bill does not require concealed-carry reciprocity among states, a provision requested by Toomey that was flatly rejected by Democrats this week during closed-door negotiations, according to several aides familiar with the talks.

There was, however, an informal agreement to vote on the measure as an amendment, according to Senate aides familiar with the talks. And it is that vote that has the gun control advocates amost worried.

The proposal preferred by the NRA and most Republicans (it was endorsed in the GOP 2012 platform) was offered up in 2009 by Sen. John Thune (R-S.D.). It would grant individuals who have a concealed-carry permit in one state the right to transfer that privilege to another state. The proposal earned 58 votes in the Senate — two short of the magic 60 needed to advance the bill to final passage — but enough to worry gun control groups.

They were further irked that year when two pro-gun-rights proposals passed with Democratic support and became law with President Obama’s signature. In May 2009, Oklahoma Republican Sen. Tom Coburn introduced an amendment that would have allowed guns to be carried in national parks. Democrats had 56 members voting (four seats were either vacant or the incumbent was too ill to vote) to 40 for Republicans. The bill got 67 ayes with 27 Democrats supporting it. (Twenty-nine Democrats opposed it.)

Later, Senate Republicans introduced an amendment to an appropriations bill funding transportation projects that would have allowed people to carry firearms on Amtrak trains. At the time, Democrats controlled 59 seats in the Senate while Republicans held 40. The amendment earned 68 ayes — 28 from Democrats.

Among the Democrats who voted for all three proposals were Majority Leader Harry M. Reid (D-Nev.) and several rural-state senators facing difficult reelection bids in 2014: Max Baucus (D-Mont.), Mark Begich (D-Alaska), Kay Hagan (D-N.C.), Mary Landrieu (D-La.) and Mark Pryor (D-Ark.). These senators have not said whether they would support similarly pro-gun-rights proposals this year.

A Thune aide said the senator still supports the plan, but wouldn’t say whether he plans to reintroduce his proposal. Republicans are considering dozens of additional amendments to prolong or upend the debate and legislation. Democrats are considered likely to settle on just a few amendments, including plans to ban military-style assault weapons and limit the size of ammunition magazines, proposals that Democrats admit are likely to fail.

To avoid amendments they don’t support, Senate Democrats could try to force 60-vote thresholds on pro-gun amendments, but such a move would most likely threaten to unwind the entire legislation in a procedural quagmire that leaves the final bill failing to muster the 60 votes needed for final passage — and also spoil the bipartisan spirit with which debate began this week.

If the Thune amendment is reintroduced this year, it would probably kill the Senate’s attempt to enact new gun laws, according to Arkadi Gerney, a senior fellow at the liberal Center for American Progress, who is closely tracking gun votes. In 2009, Gerney lobbied against the Thune proposal on behalf of New York Mayor Michael Bloomberg’s group, Mayors Against Illegal Guns.

Richard Feldman, a former NRA lobbyist and head of the Independent Firearm Owners Association, agreed that the Thune proposal would scuttle the bill.

“Part of me likes it, but from a legislative strategy, it becomes a killer amendment,” Feldman said. “The same votes that would be for background checks, lots of them would vote no on the final bill” if the Thune proposal is included.

In hopes of saving the overall bill, Democrats and gun control groups are working on a plan to counter expected strong pressure from the NRA to pass the reciprocity plan in the coming weeks. To woo red state Democrats, the gun control crowd is preparing back-up legislation that would use the Thune language of reciprocity but add some additional requirements. For example, the secondary amendment to Thune might add language that would only allow reciprocity to apply in cases where a state meets minimum standards for granting a permit.

Gun control legislators and lobbyists are also talking about adding a poison pill, such as a requirement that state permit holders seeking to extend their rights to other states be identified in a national database that could be accessed by law enforcement — an idea strongly opposed by many gun rights activists.

Paul Kane contributed to this report.

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