Without offering details, Romney said that Obama would like to erode gun owners' rights.
"We need a president who will enforce current laws, not create new ones that only serve to burden lawful gun owners," Romney told thousands of NRA members in St. Louis for their annual convention. "President Obama has not. I will."
Obama has said relatively little about firearms, deeply disappointing gun-control groups. Obama campaign spokesman Ben LaBolt said the president's record "makes clear the he supports and respects the Second Amendment, and we'll fight back against any attempts to mislead voters."
The gathering of gun enthusiasts comes as Romney, the presumptive GOP nominee, is trying to woo conservative groups to consolidate his base after fending off challengers on his right. His relationship with gun owner groups is uneasy.
Running for the Senate in 1994, Romney said: "I don't line up with the NRA." A decade later he became a lifetime NRA member.
The NRA convention is a must-do for any Republican candidate. But this week it interrupted a new campaign narrative that Romney would like to extend: the view that top Democrats look down on stay-at-home moms.
Romney used the NRA setting, in the domed arena where the St. Louis Rams play football, to make his first public comments on the topic. He brought a central player and key supporter: his wife, Ann, who stayed home to raise their five sons.
Democratic activist Hilary Rosen set off the tempest Wednesday by telling CNN that Ann Romney never worked a day in her life. The ensuing uproar knocked Democrats off their message that Republicans are insufficiently concerned about women's rights, including access to birth control.
In St. Louis, Mitt Romney began his 24-minute speech by calling his wife "a hero" and "my sweetheart," adding: "I happen to believe all moms are working moms."
Ann Romney praised working fathers as well as mothers, then left the stage to her husband and his appeal to the gun-rights group.
Mitt Romney told the group: "We need a president who will stand up for the rights of hunters, sportsmen, and those seeking to protect their homes and their families. President Obama has not; I will."
Asked for details to support the claims, Romney's campaign said Obama has appointed judges, including Supreme Court Justices Elena Kagan and Sonia Sotomayor, who have supported moves such as placing temporary limits on importing semiautomatic assault weapons. The campaign said Attorney General Eric Holder has not adequately backed people's rights to own and use firearms.
But gun-control groups such as the Brady Center to Prevent Gun Violence have expressed dismay over the lack of attention to their concerns. In its most recent assessment, in 2010, the group flunked on Obama on all seven issues it deemed important.
Campaigning in 2008, Obama said: "I believe in people's lawful right to bear arms. ... There are some commonsense gun safety laws that I believe in. But I am not going to take your guns away."
Dan Gross, president of the Brady group, said he is happy that Obama recently spoke in support of the family of Florida shooting victim Trayvon Martin and called for more national dialogue on gun violence following last year's shooting of then-Rep. Gabriel Giffords, D-Ariz., which killed six people.
"Our disappointment is that his voice is really yet to be heard in that conversation," Gross said. He said Romney is pandering to the NRA, a group he accuses of abetting the killings of thousands of people.
Neither Romney nor other NRA speakers -- former presidential contender Rick Santorum and still-trying candidate Newt Gingrich were among them -- alluded to the high-profile Martin case. A volunteer neighborhood watchman has been charged with second-degree murder for fatally shooting the unarmed teenager, who was walking in a gated community.
The NRA strongly backed Florida's "stand your ground" law, at the heart of the unfolding case.
Romney spokeswoman Andrea Saul said Romney "believes that efforts to craft `stand your ground' legislation should be left up to the individual states."
When Romney ran successfully for Massachusetts governor in 2002, the NRA gave his Democratic opponent a higher rating on gun-rights issues but made no endorsement.
Massachusetts quadrupled its gun-licensing fee while Romney was governor. He signed a law that made permanent a ban on assault-type weapons, although it was coupled with some measures backed by gun-rights groups.
As he was considering his first presidential run in 2006, Romney became a lifetime NRA member.
Romney drew snickers in 2008 by claiming he sometimes hunts "small varmints." He showed more humility and humor last month in Alabama, where he said he hoped to go hunting with a friend who "can actually show me which end of the rifle to point."
Gingrich, who addressed the NRA convention after Romney, said the United Nations should adopt a treaty "to extend the right to bear arms to every person on the planet." Such "human rights," he said, would reduce rapes and child killings worldwide.