Jim Cavanaugh, special agent in charge of ATF's Nashville field office, said the two men planned to shoot 88 black people and decapitate another 14. The numbers 88 and 14 are symbolic in the white supremacist community.
The men also sought to go on a national killing spree after the Tennessee murders, with Obama as its final target, Cavanaugh told The Associated Press.
"They said that would be their last, final act — that they would attempt to kill Sen. Obama," Cavanaugh said. "They didn't believe they would be able to do it, but that they would get killed trying."
An Obama spokeswoman traveling with the senator in Pennsylvania had no immediate comment.
The men, Daniel Cowart, 20, of Bells, Tenn., and Paul Schlesselman 18, of West Helena, Ark., are being held without bond. Agents seized a rifle, a sawed-off shotgun and three pistols from the men when they were arrested. Authorities alleged the two men were preparing to break into a gun shop to steal more.
Attorney Joe Byrd, who has been hired to represent Cowart, did not immediately return a call seeking comment Monday.
Cowart and Schlesselman are charged with possessing an unregistered firearm, conspiring to steal firearms from a federally licensed gun dealer, and threatening a candidate for president.
The investigation is continuing, and more charges are possible, Cavanaugh said.
The court records say Cowart and Schlesselman also bought nylon rope and ski masks to use in a robbery or home invasion to fund their spree, during which they allegedly planned to go from state to state and kill people.
For the Obama plot, the legal documents show, Cowart and Schlesselman "planned to drive their vehicle as fast as they could toward Obama shooting at him from the windows."
"Both individuals stated they would dress in all white tuxedos and wear top hats during the assassination attempt," the court complaint states. "Both individuals further stated they knew they would and were willing to die during this attempt."
Cavanaugh said there's no evidence — so far — that others were willing to assist Cowart and Schlesselman with the plot.
He said authorities took the threats very seriously.
"They seemed determined to do it," Cavanaugh said. "Even if they were just to try it, it would be a trail of tears around the South."