Asked at a news conference if he would be prepared to debate Capriles, Chavez replied: "I'd be ashamed because what you have there is nothing ... I'd have liked to face a heavyweight, not a non-entity."
Since Capriles, a center-left young state governor, won the opposition coalition's primary in February, Chavez has not mentioned him by name, preferring to use a stream of insults including "pig" and "loser".
Capriles has sought to focus his campaign on Venezuelans' daily problems - crime, unemployment and stuttering social services - rather than be drawn into a rhetorical fight.
1. Chavez is allegedly on strong pain meds, owing to bone pain.
He insists he's better but refuses to address the state of his health beyond that.
His actual condition and capacity to carry on with his typically torrid pace of campaigning are still shrouded in mystery. News reports that he has only a few months to live continue to surface, as they have periodically since he announced his cancer last June.
One source close to Chavez's medical team told Reuters his condition was "delicate" and he had started to experience strong pain in one leg last month due to the illness' progression.
Reporters, including one prominent opposition journalist, have said Chavez is taking powerful opiates to ease the pain of what they said was cancer that had spread to his bones.
The cancer has given an extraordinary backdrop to the election and is eclipsing all other major factors in the campaign - including an unprecedented push by the opposition to rally behind a single candidate.