Despite assurances from top government officials and close relatives that Chavez is recuperating following surgery more than two weeks ago, the president's silence and seclusion since the operation have spurred growing speculation about how ill Chavez may be.
Opponents say Chavez and his aides should be more straightforward.
"The uncertainty regarding Hugo Chavez's health and considerable speculation over the real illness affecting him reveal the government's serious constitutional violations," said Miguel Angel Rodriguez, an opposition lawmaker.
Under Venezuela's Constitution, Chavez must "give us the diagnosis, talk to us about the treatment and answer questions," Rodriguez said in a statement.
Venezuelan officials have said Chavez is recuperating, but have provided few details.
Chavez's Twitter stream has been active, but it has not provided any information about his health. Three messages appeared within 30 minutes Saturday afternoon, including one mentioning visits by Chavez's daughter Rosines and grandchildren.
"Ah, what happiness it is to receive this shower of love!" the Twitter message read. "God bless them!"
Nobody has heard Chavez speak publicly since he told Venezuelan state television by telephone on June 12 that he was quickly recovering from the surgery he had undergone two days earlier for a pelvic abscess. He said medical tests showed no sign of any "malignant" illness.
It remains unclear when he will return to Venezuela.
Chavez's mother, Elena, wished her son a speedy recovery on Sunday.
"May the power of God heal him and bring him to me," she told state television.
The vice president must take the president's place during temporary absences of up to 90 days, according to the constitution. Some opposition politicians have suggested that Vice President Elias Jaua should replace Chavez until he recovers, a move that Jaua has ruled out.
If Chavez were to relinquish power, some analysts believe his political movement would crumble or split.
"No one else is really ready to step in and take charge," said Michael Shifter, president of the Inter-American Dialogue, a Washington-based think tank. "The current situation shows how precarious one-man rule is: Everything hinges on the whims of a single individual."
"A search for a successor to Chavez would significantly scramble the country's politics," Shifter added. "A fierce power struggle within Chavismo would almost certainly ensue."
Infighting also would likely break out within Venezuela's loosely knit opposition, which plans to hold a primary to pick a presidential candidate for next year's election.
"The opposition would also be thrown off balance," Shifter said. "Their single-minded focus on Chavez has kept them more united in recent years."
