Sen. Tom Coburn, R-Okla., sponsored the measure, which would restore a Bush administration policy allowing loaded weapons in national parks.
The relaxed gun rules, introduced on January 9, allowed visitors to carry a loaded gun into a park or wildlife refuge as long as the person had a permit for a concealed weapon and the state allowed concealed firearms. Previously, guns were required to be unloaded and locked away, such as in a car trunk.
In March U.S. District Judge Colleen Kollar-Kotelly blocked the rule, calling the Interior Department's rule-making process "astoundingly flawed" and said officials failed to evaluate (as required by law) the possible environmental impacts of allowing loaded weapons into the parks.
The Obama administration accepted the ruling, saying that the Interior Department would conduct a full environmental review.
Sen. Coburn then attached the gun amendment to an unrelated Senate bill imposing restrictions on credit card companies.
To the surprise of many, the amendment easily passed, winning support from 67 senators - including 27 Democrats. Among those who voted "yes" was Senate Majority Leader Harry Reid of Nevada, who had blocked Coburn's amendment from coming to the Senate floor for more than a year.
The credit card bill passed 90-5 Tuesday. The measure now goes to the House, where approval is expected.
Reid Spokesman Jim Manley said the Senator is a strong supporter of the Second Amendment, adding that the guns in parks issue was a major concern for many Nevadans.
"The rules that apply to our federal lands are felt acutely in Nevada, where 87 percent of the state's land is managed by federal agencies," Manley said.
The Brady Campaign to Prevent Gun Violence, which sued to block the Bush policy, called the Senate vote reckless. The group called on President Barack Obama to demand that the gun provision be stripped from the credit card bill.
"Families should not have to stare down loaded AK-47s on nature hikes," said Brady campaign president Paul Helmke. "The president should not remain silent while Congress inserts reckless gun policies that he strongly opposes into a bill that has nothing whatsoever to do with guns."
Helmke and other critics, including environmental groups, park rangers and the Humane Society, say the Coburn amendment goes further than the Bush administration policy that briefly allowed loaded handguns in national parks and refuges. The measure would allow individuals to openly carry rifles, shotguns and even semiautomatic weapons on ranger-led hikes and campfire programs at national parks, the groups said.
Coburn said the gun measure protects every American's Second Amendment rights and also protects the rights of states to pass laws that apply to their entire state, including public lands.
"Visitors to national parks should have the right to defend themselves in accordance with the laws of their states," Coburn said.
Last year seven former directors of the National Park Service - including the Bush administration's first park chief - signed a letter opposing a relaxation of gun restrictions in parks. The former park officials said the stricter regulations had helped make national parks among the safest places in America.
But the National Rifle Association and other gun-rights advocates pushed to loosen the gun rules.