By Andrew Cawthorne and Daniel Wallis
CARACAS (Reuters) - Venezuelan President Hugo Chavez's election rival wished him a fast recovery on Wednesday while allies closed ranks behind the "comandante" whose announcement of new surgery in Cuba revived doubts over his future.
The 57-year-old socialist will head to Havana in the coming days for an operation on a possibly malignant lesion in the same area of his pelvis where he had a tumor removed last year.
With less than eight months before a presidential vote in the South American OPEC nation, Chavez's imminent departure for more delicate medical treatment has heaped uncertainty and emotion onto an already charged political atmosphere.
"As one of God's people, I wish my rival a successful operation, a quick recovery and a long life," said the opposition's presidential candidate Henrique Capriles in a carefully-measured comment via Twitter.
Capriles, who won the Democratic Unity coalition's primary this month, does not want to be seen exulting in Chavez's health problems and is trying to focus the campaign on issues such as crime and unemployment rather than a head-to-head battle.
"I'M NOT IMMORTAL"
Chavez's new surgery could hardly have come at a worse time for the government as Capriles, a 39-year-old state governor who exudes youth and energy, enjoys momentum from his primary win.
The president's allies, some of whom had denied rumors of a downturn in Chavez's health last weekend, did a quick U-turn, calling for calm in Venezuela when he returns to Havana.
"Our message is for unity and faith that President Chavez will triumph over this latest obstacle life has put in his way," Information Minister Andres Izarra said. "His connection with the people will help him overcome any health problem."
Underlining the secrecy surrounding Chavez's condition, Izarra was among those virulently condemning rumors he had gone to Cuba for treatment during Venezuela's long carnival weekend.
Announcing that he had been to Havana and would need to return by the weekend, Chavez insisted his new operation would be smaller than the first cancer surgery in June and that he would be fine for the presidential race.
Less reassuringly for supporters, however, he also said: "I'm a human being, I'm not immortal."
Venezuela was awash with speculation and gossip on Wednesday. In the absence of medical details, theories among Venezuelans of all classes and political opinions varied wildly.
At one end of the spectrum, some think Chavez has invented his health problems to court sympathy and create an artificial sense of conquering problems. At the other, some have him dying by the end of the year.
Cancer experts said the word chosen by Chavez - "lesion" - was deliberately opaque and could cover a wide variety of medical scenarios. Chavez himself said the lesion was probably malignant and that he would be under the knife by the weekend.
Critics say he should name a temporary replacement, but that is unlikely given he avoided that and governed from his hospital room in Havana during extended absences last year.
Having dominated the political stage for so long, Chavez has no clear successor. Vice-president Elias Jaua, who would theoretically take over if Chavez is incapacitated, lacks big clout in the ruling Socialist Party or among the military.
With investors always looking for signs of a more market-friendly government, Venezuelan bond prices rose slightly after the news of Chavez's surgery.
"In our opinion, the probability that Chavez could not be a candidate has increased after this announcement," Barclays Capital said in a research note.
As always, Chavez was upbeat in his comments on Tuesday, saying there was no metastasis and nobody should be alarmed.
One medical source close to the team treating Chavez in Venezuela said he had been suffering a tumor lysis, or cell breakdown, which carried symptoms including a high fever.
Prior to Tuesday's announcement, opinion polls gave Chavez an edge over Capriles, thanks partly to a huge program of new state spending on social projects. But about a third of Venezuelans remain undecided, so everything is to play for.
"It's possible that the first reactions are of solidarity," local pollster Luis Vicente Leon said of the potential impact of Chavez's health on public opinion going forward.
"But like we saw last year, that momentum gradually fades."
(Additional reporting by Mario Naranjo; Editing by Kieran Murray)