"Hillary was right: You can't trust Obama with your guns," the association's Political Victory Fund said in a recent mailing.
The endorsement reflects the complicated role that gun control is playing in the 2008 campaign.
For voters who care about the issue, the most unambiguous record belongs not to McCain, but his running mate.
Alaska Gov. Sarah Palin is not only a gun owner and an NRA member, but a proud hunter who is unapologetic about supporting aerial wolf hunting.
From there, things get kind of gray.
Obama's running rate, Sen. Joe Biden of Delaware, favors gun control but is a gun owner himself - he has three shotguns.
Referring to Obama, Biden recently told a Virginia audience, "He tries to fool with my Beretta, he's got a problem." He was referring to a weapon sold by Beretta, a company that makes a wide variety of firearms.
Neither McCain nor Obama owns a gun.
Obama, a former constitutional law professor, joins McCain in saying he supports the Second Amendment's right to bear arms. And both said the Supreme Court was correct in June when it clarified - for the first time in the country's history - that the Second Amendment gave gun ownership rights to individuals, not just a militia.
But that case also exposed a fissure between the two White House contenders.
McCain, reflecting the more conservative principles of the GOP, said the ruling is only the first step toward ensuring gun rights.
While he supports the gun-show checks and advertising restrictions, he voted against the 1994 assault weapons ban and believes gun manufacturers should not be held liable in civil lawsuits for crimes committed with their products.
The Arizona senator said last year: "I strongly support the Second Amendment. And I believe the Second Amendment ought to be preserved - which means no gun control."
Obama joined with fellow Democrats in voting to leave gun-makers and dealers open to suit. As an Illinois state lawmaker, he also supported a ban on all forms of semiautomatic weapons and tighter state restrictions generally on firearms. He argues local communities should be able to take the steps they feel are necessary to reduce crime.
Those positions helped him win the endorsement of the Brady Campaign to Prevent Gun Violence.
"The Obama-Biden ticket will work with law enforcement, gun violence victims and ordinary citizens who want to do more to protect themselves, their families and their communities by making it harder for dangerous people to get dangerous weapons," said Paul Helmke, president of the group named after Jim Brady. Brady was press secretary to President Reagan when he was gravely wounded by a gunman who tried to assassinate Reagan in 1981.
A rival gun rights group, the American Hunters and Shooters Association, has endorsed Obama and accuses the NRA of backing McCain for partisan rather than philosophical reasons.
"The NRA is holding Obama accountable for votes taken 10 years ago but giving McCain a pass for his actions within the past eight years," said the group's president, former Washington Redskins player Ray Schoenke, who has cut radio ads for Obama.
Ohio Gov. Ted Strickland, a Democrat, has also tried to reassure voters in his battleground state, saying last week that gun owners "have nothing to fear from Barack Obama."
McCain generally does not raise the issue on the campaign trail. The former naval aviator was trained to use a pistol and carried one with him in combat but hasn't used one since leaving the military.
In May, however, speaking to the NRA's annual convention in Kentucky, McCain leveled a double-barreled attack on Obama and Sen. Hillary Rodham Clinton, who was then still in the Democratic nomination race.
"If either Sen. Clinton or Sen. Obama is elected president, the rights of law-abiding gun owners will be at risk, my friends - and have no doubt about it," McCain said.
While his McCain-Feingold law inhibits special-interest fundraising, he told the group, "Those disagreements do not detract from my long record of support for the Second Amendment and the work we have done together to protect the rights of gun owners."
Obama, meanwhile, angered both gun owners and blue-collar workers with his comment during a San Francisco fundraiser that bitter voters in rural Pennsylvania "cling" to guns and religion because of their economic frustrations.
Last week, during an appearance in Lebanon, Ohio, he, too, tried to reassure gun owners.
"I believe in the Second Amendment," he declared. "I believe in people's lawful right to bear arms. I will not take your shotgun away. I will not take your rifle away. I won't take your handgun away."