CARACAS, Venezuela (AP) -- Venezuelan President Hugo Chavez's cancer relapse and his sudden announcement that he will undergo a fourth cancer-related surgery in Cuba have thrown the country's future into question, and his designated political heir has begun trying to fill the void.
Underlining the gravity of the situation, Vice President Nicolas Maduro broke into tears on Monday at a political rally hours after Chavez flew to Havana.
"Chavez has a nation, he has all of us, and he'll have all of us forever in this battle," said Maduro, who wiped away tears while speaking to supporters. "Even beyond this life, we're going to be loyal to Hugo Chavez."
Maduro called for the president's supporters to rally behind his candidates in upcoming gubernatorial elections on Sunday, and he also inaugurated a new cable car system in a poor neighborhood. Maduro, who spoke passionately and wore the red of Chavez's socialist movement, seems set to take on a larger role as the president's chosen successor.
Chavez said for the first time on Saturday that if he suffers complications, Maduro should take over for him and should be elected president to continue his socialist movement.
Analysts say Maduro faces monumental challenges in trying to stand in for his mentor and hold together the president's diverse "Chavismo" movement, while also coping with economic problems that are weighing on the government.
Maduro may inherit political power, "but he definitely can't inherit the charisma" of Chavez, said Luis Vicente Leon, a pollster who heads the Venezuelan firm Datanalisis. He said that during his nearly 14 years in office Chavez has been the glue that has held together groups from radical leftists to moderates, as well as military factions.
Leon said it's unclear if Maduro has what it takes to hold the movement together if Chavez dies. "Internal divisions could make the revolution unstable in the future," Leon said.
Political analyst Vladimir Villegas, who has known Maduro since his adolescence, said the vice president's experience years ago as a public transit union leader will probably help him in the difficult task of mediating between different groups of Chavez allies.
Maduro is considered to belong to a radical leftist wing of Chavez's movement that is closely aligned with Cuba's communist government. But Villegas said he thinks Maduro will know how to contain his radicalism for practical purposes.
"The priority will be the preservation of political stability, for which it will be necessary to begin negotiating with internal groups and even with the opposition," said Villegas, who hosts a radio program. "This situation is going to force him to proceed with caution."
Villegas also said that Maduro will need to "hold on to the trust if the Chavista base and to neutralize the fractions that are going to bet... on him not doing well."
Before leaving for Havana early Monday, Chavez met with military commanders at the presidential palace and promoted his defense minister, Diego Molero, to the rank of admiral in chief. Chavez showed Molero and other military commanders a golden sword that once belonged to independence hero Simon Bolivar.
Holding the sword, Chavez told the officers that he fully trusts them. He also warned of potential conspiracies by enemies, both foreign and domestic.
"I'm totally sure that our homeland is safe," Chavez told them. He urged them "not to give in to intrigue."
Chavez announced that his cancer had reappeared and named Maduro as his chosen successor during a quick weekend visit to Caracas after spending 10 days in Cuba for treatment. He said he wanted to return to deliver his message to the nation, and his appearance after a prolonged absence allowed him to send a clear directive to his movement that it should follow Maduro if cancer cuts short his presidency.
Many in Venezuela have interpreted his message as indicating that he now faces long odds.
Video of his departure, played hours later on state television, showed Chavez raising a fist as he climbed the stairs alone. From top of the stairs, he waved and shouted "Long live our homeland!"
Also visible in the doorway of the plane were his eldest daughter, Rosa, and a grandson.
The 58-year-old president won re-election in October and is due to be sworn in for a new six-year term on Jan. 10. If Chavez were to die, the constitution says that new elections should be called and held within 30 days.
Chavez said on Saturday that if such new elections are held, Maduro should be elected president in his place.
In the meantime, Maduro is helping to lead a government with serious economic problems including a swelling budget deficit and a currency that has a rapid drop in black market trading.
The vice president is also actively campaigning ahead of this weekend's state elections, telling supporters on Monday that when casting ballots "we're there with Chavez."
Chavez plans to undergo his third operation to remove cancerous tissue in about a year and a half. An initial surgery for a pelvic abscess in June 2011 helped reveal he had cancer. He has also undergone chemotherapy and radiation treatments.
Chavez said in July that tests showed he was cancer-free. But he had recently reduced his public appearances and on Nov. 27 returned to Cuba saying he would undergo hyperbaric oxygen treatment. Such treatment is regularly used to help heal tissues damaged by radiation treatment.
He said that while in Cuba tests found a return of "some malignant cells" in the same area where tumors were previously removed. Chavez said he will undergo surgery in the coming days, but it's not clear how soon.
As Chavez arrived in Havana in the early morning darkness, he received a typical welcome from Cuban President Raul Castro, who hugged him and smiled for the cameras.
But he also received a last-minute visit from Ecuadorean President Rafael Correa, who flew to Cuba on Monday.
"He has a very grave health problem," Correa told reporters at Havana's international airport. "We came to give him a hug in the name of the Ecuadorean people. ... He is not alone."
Associated Press writers Ian James in Caracas and Andrea Rodriguez in Havana contributed to this report.