Defenders say it will create a driver’s-license-style system for gun packing.

The big gun policy battles of the Obama years — over expanding background checks, and closing the so-called terror gap — have been shoved aside in Donald Trump’s Washington, where a major fight looms over the National Rifle Association’s top legislative priority: federal legislation that would create national reciprocity for concealed-carry licenses, requiring every state to accept the permits of the other 49, regardless of differences in standards for who is eligible to bring a hidden gun into public spaces.

Bills establishing national concealed-carry reciprocity, as the policy is known, are already filed in both chambers of Congress. As NRA members gather for their annual convention in Atlanta this weekend, look for the group to kick off the blitz for a policy that Trump has promised to deliver, even as gun safety groups are rallying to deny the NRA the victory.

Here’s what you need to know about an idea that supporters say will simply create a driver’s-license-style system for carrying a gun, and that critics call a recipe for shootouts between tourists on busy city streets.

What is “national concealed-carry reciprocity?”

Licenses to carry concealed guns are issued by state and local authorities, who also decide which outside licenses they’ll accept. The policy the NRA is championing would change that, by requiring each state to recognize concealed-carry permits issued by every other state. It would not create a national standard for concealed carry.

How much do states’ individual concealed-carry standards differ?

Quite a lot. Massachusetts, California, New York, New Jersey, Maryland, Delaware, and Hawaii require applicants to show a specific need for a gun. (Many cities, like New York, also impose their own restrictions on gun carrying, which are often equally, if not more, strict than the toughest states.) Rhode Island and Connecticut also give permit-issuing authorities broad discretion to reject applicants — for example, if authorities doubt the character of the permit seeker. Thirty states have “shall issue” laws, which limit local discretion over permits and mandate that issuing authorities give concealed-carry permits to anyone who applies and meets any other criteria. In those states, most people who want a permit get one.

Eleven states allow their citizens to carry guns without obtaining a permit, including two, New Hampshire and North Dakota, that changed their laws this year. In those states, anyone who can legally possess a gun is allowed to carry it concealed without having to seek a license or meet training requirements. (These states also still issue concealed carry permits if gun owners want them.) Vermont, a permitless carry state, does not issue licenses at all.

What’s the NRA’s gripe with the current system?

The NRA and advocates of concealed-carry reciprocity argue that varying state laws create a “confusing patchwork” that often makes “accidental criminals” of gun owners who travel with weapons, but don’t know local laws.

They support a national concealed-carry policy as a modest streamlining of existing statutes. A metaphor that proponents often cite is state-issued driver’s licenses, which are valid nationwide. They say they want concealed gun permits to work the same way.

“If you can drive in Texas, you can drive in New York and follow New York laws,” said Senator John Cornyn, a Texas Republican who is sponsoring a concealed-carry reciprocity bill in the Senate.

Drivers licenses and gun permits… Is that an accurate comparison?

Not totally. States honor one another’s driver’s licenses under a voluntary agreement. There’s no federal mandate. States also have fairly uniform requirements for issuing driver’s licenses. If you’re renting a car to someone with a New York license who is visiting Texas, you can be pretty confident the New Yorker has passed a road test. Guns are different. At least 26 states will issue a gun permit to someone without requiring that person to ever actually shoot a gun.

Opponents of the reciprocity movement argue that allowing people from states with minimal licensing requirements to carry their concealed guns in any other state would impose the lowest common denominator nationally. States with strict laws would have to allow people from states like Wyoming, Mississippi or Vermont — which allow residents to carry guns without a permit — to carry guns.

What’s really going on here?

The push for a national concealed-carry policy pits gun-rights advocates against the states and cities looking to enact strict laws backed by local voters. The NRA and its allies say they just want to make gun laws simpler. Critics see the effort as an example of the group’s strategy of working not to simply defend the interests of gun owners but to topple any regulation that suggests firearms are too dangerous to be allowed in certain areas.

“What the NRA really wants to do on concealed carry is overturn these clear-cut restrictions on people carrying guns,” said Adam Winkler, a law professor at the University of California at Los Angeles who studies firearm laws. “They want to make guns very much a prominent part of American life — guns on college campuses; guns in bars, churches, wherever then can get them.”

So there’s national reciprocity legislation sitting in Congress. What exactly would it do?

There are actually two such bills. One introduced in the Senate by Cornyn, and another introduced in the House by Representative Richard Hudson, a North Carolina Republican. Both measures would allow people who legally carry guns in their home state to do so in every other state — including people who reside in states that don’t require permits. Both bills specify that the law would apply not only to all concealed-carry permit holders but to anyone allowed to carry a gun where they live.

Hudson’s bill, however, is more aggressive than Cornyn’s. It would also mandate that residents of a given state could get a license from another state in order to carry their gun at home. California residents could turn to Utah, where permits are especially easy to get, to duck tougher local permitting laws. Maryland residents could get Virginia licenses that would work at home.

Hudson’s bill would also undermine the federal Gun Free School Zones Act by exempting permit holders from its requirements. Finally, the bill would allow people whose permits are questioned by local law enforcement officials to sue police departments and individual officers for legal fees and damages. The lawsuit provision appears aimed at keeping police officers from questioning the gun permits of out-of-state residents, said Lindsay Nichols, a senior attorney at the Law Center to Prevent Gun Violence.

Who opposes national concealed-carry reciprocity?

Law enforcement officials are one prominent source of opposition. They argue that the bills would complicate, not simplify, their jobs. Baltimore County Chief of Police Jim Johnson, the chairman of the National Law Enforcement Partnership to Prevent Gun Violence, told The Trace that the reciprocity proposals put police officers in danger, forcing them “at 2 o’clock in the morning” to confront armed out-of-staters with unfamiliar credentials.

Groups like Johnson’s are likely to play a big role in opposing the national reciprocity legislation; their involvement helps counter claims that opposition to the bills comes largely from coastal liberals. Prosecutors also represent a similarly potent source of resistance.In a March Wall Street Journal op-ed, New York Police Commissioner James O’Neil and Manhattan District Attorney Cyrus Vance Jr. argued that a national concealed-carry policy would represent “a dangerous and unwarranted interference with state and city laws.”

Gun safety organizations are also gearing up for a major push against the proposal. Everytown for Gun Safety, which provides a portion of The Trace’s funding, has vowed to spend heavily against lawmakers who support the bill. Borrowing a page from the NRA, the group says it will score members of Congress based on their votes for or against the reciprocity measure.

Aren’t Republicans usually for state’s rights?

Not when it comes to gun rights. Advocates of national concealed carry, including Representative Hudson, have argued that their proposals don’t impose on states because they apply only to states that choose to allow some concealed carry. But that’s a misleading claim: The bill would apply to every state because all allow at least some segment of the population to carry guns. Supporters of concealed-carry reciprocity prioritize an expansive view of gun rights over federalism.

Will it pass?

A national concealed-carry bill could easily pass the House, where Hudson’s bill had 188 co-sponsors as of mid-April. President Trump has said he would happily sign it. But it faces an uphill battle in the Senate, where Republicans would need 60 votes — including eight from Democrats — to overcome a filibuster from Democrats, which, on this bill, is considered inevitable.

In 2013, in a vote that received limited notice, 13 Democrats voted for a version of Cornyn’s bill. Seven of those senators — Jon Tester of Montana; Joe Manchin of West Virginia; Joe Donnelly of Indiana; Heidi Heitkamp of North Dakota; Martin Heinrich and Tom Udall of New Mexico; and Mark Warner of Virginia — remain in office. Five of those lawmakers are up for re-election in 2018, along with several other Democrats in gun-friendly states. The NRA and its allies in the Republican leadership will be looking to put maximum pressure on red-state Democrats to back the bill, or at least make opposing it more politically costly.

Still, Democrats remain confident they can block the measure. Senate aides said that some Democrats who backed the bill in 2013 when it had no chance of becoming law would likely now oppose it.

Chris Cox, head of the NRA’s lobbying arm, has stopped short of predicting passage.

“It’s going to be a hard fight,” Cox said in January.