Chris Cox, executive director of the National Rifle Association's political action committee said membership is growing at record levels and gun sales are up as much as 50 percent since Obama won, The Hill reported.
During a weekend news conference, Obama said he believes "in common-sense gun safety laws and I believe in the Second Amendment."
"He says gun owners shouldn't be worried, but clearly gun owners are worried," Cox told the Washington publication.
In campaigning against Republican nominee Sen. John McCain of Arizona, Obama ran television ads promising gun owners they should not worry about his administration infringing on Second Amendment rights.
But Cox and other gun rights groups say they expect Obama to back off his campaign promise.
"We have no doubt that he's going to break his promise to the American people that he made during the campaign, and we're prepared to fight him every step of the way," Cox said, declining to say how much money the NRA has to wage such a battle.
John Velleco, federal affairs director for Gun Owners of America, told The Hill his group is preparing to target Congress to prevent gun-control legislation from reaching the White House.
"We are going to try to keep the heat on the members of Congress who claim to be pro-gun but have not yet been tested and try to prevent them from sending President Obama anything he can sign," Velleco said.