Nicolas Maduro said the plan was to blame his opponent's murder on the OPEC nation's government and to "fill Venezuelans with hate" as they prepare to vote following the death of socialist leader Hugo Chavez.
"I call on President Obama - Roger Noriega, Otto Reich, officials at the Pentagon and at the CIA are behind a plan to assassinate the right-wing presidential candidate to create chaos," Maduro said in a TV interview broadcast on Sunday.
Maduro, who is Chavez's preferred successor as president, said the purpose of the plot was to set off a coup and that his information came from "a very good source."
Maduro first mentioned a plot against his rival, Henrique Capriles, last week. He blamed it on former Bush administration officials Roger Noriega and Otto Reich.
The United States denied the claim with the State Department spokesman Patrick Ventrell saying, "We categorically reject allegations of U.S. government involvement in any plot to harm anyone in Venezuela".
During his 14 years in power, Chavez often denounced U.S. plots against him and his revolution.