By Daniel Wallis and Brian Ellsworth
CARACAS (Reuters) - He uses Hugo Chavez's bombastic language, brandishes the constitution and showers opponents with vitriol at every turn.
But Venezuelan Vice President Nicolas Maduro is struggling to replicate the extraordinary charisma of his boss, who is battling to recover from cancer surgery in Cuba.
Named as heir apparent by Chavez just before the president returned to Havana for his fourth surgery in December, the 50-year-old former bus driver has become the face of the socialist government in South America's top oil exporter.
Though diplomats say he is easy-going in conversations behind closed doors, in public he can be just as caustic and combative as Chavez, the former soldier who has led Venezuela for the past 14 years.
Having made few notable decisions so far during his three-week tenure as stand-in president, Maduro has not offered many clues as to how he would lead if he were at the helm of a post-Chavez Venezuela.
"Every day, Hugo Chavez makes us better patriots and better Venezuelans. He has created a generation of revolutionaries and blazed a trail for us," Maduro told one rally last week, lionizing Chavez in gushing and quasi-messianic terms.
"We swear on the constitution that we will be loyal to his leadership ... that we will confront all U.S. imperialist aggression, and dismantle the lies of the bourgeois traitors."
Chavez, 58, has not been seen in public nor heard from since early December, adding to speculation that his polarizing rule could be coming to an end. Late on Sunday, Maduro said the president was suffering a third set of complications after surgery, and remained in a "delicate" state.
If Chavez died or had to step down, new elections would be held within 30 days. Maduro, who also served as foreign minister for the last six years, would be the candidate of the ruling Socialist Party, or PSUV.
Sometimes dressing casually like Chavez in a track suit to address supporters, he has taken on the president's hectoring style too, slamming opposition leaders as "miserable traitors."
His day-to-day-activities have been dominated by swearing-in ceremonies for governors elected in a regional vote in which the PSUV won 20 of 23 state governorships.
In one particularly vituperative attack, he excoriated opposition candidate Henrique Capriles for having the nerve to campaign in Chavez's hometown of Sabaneta during his failed presidential bid earlier this year.
"He dared profane the sacred revolutionary earth of Sabaneta," Maduro said at the inauguration of Chavez's older brother Adan as governor of their home state, Barinas.
"He came here to offend the people. He had the gall to say he would win even in Sabaneta, the bourgeois rich kid," Maduro said, spitting out the final phrase like a playground insult.
Several times he has sounded choked up with emotion, and he cried as he told one crowd the president could count on them forever. "Beyond this life we will be loyal to Hugo Chavez!"
He has peppered his speeches with references to a state-backed campaign called "I am Chavez." During the inauguration of one public transport project he strayed into an awkward metaphor by saying: "Chavez is the cable car."
But his daily appearances on state television have lacked the relaxed, man-of-the-people charisma of his boss, who was always ready to hug a child or crack a joke with workers during his many tours of building sites, hospitals and oil refineries.
The opposition has called him a "poor copy" of Chavez.
'HE CHOSE MADURO'
Maduro has made no major decisions despite a consensus among business leaders and economists that Venezuela desperately needs a currency devaluation to shore up a yawning fiscal deficit and ease import bottlenecks caused by a shortage of hard currency.
He has made no apparent use of ample powers granted by Chavez this month that include authorizing debt issues and approving congressional budget allocations to ministries.
At best, he has offered vague hints of policy changes, including a veiled suggestion to limit costly subsidies for public services such as electricity and a one-sentence comment that Venezuela's exchange control system could be "improved."
When the nation's main business chamber, Fedecamaras, applauded that statement, within a day Maduro responded: "We're not going to give dollars to Fedecamaras. What this revolution is going to give them is headaches."
His speeches suggest any Maduro candidacy would be driven by the huge emotional outpouring from supporters if Chavez were to leave office or die, whipped up with pledges to follow the president's "revolutionary" course.
Political leaders who have worked with him say his experience as a union leader and diplomat could help him build ties at home and abroad. Any policy difference between Maduro and Chavez, however, would likely only become clear well after he became president.
Sensing another chance to win power just months after being beaten by Chavez at October's presidential vote, the opposition is keen to exploit any signs of a rift within "Chavismo."
Many opposition supporters suspect there are hidden tensions between Maduro and Diosdado Cabello, another powerful Chavez ally from the PSUV's military wing who has a key role as leader of the National Assembly.
There have been few indications of a split.
But Maduro and Cabello were at pains to stress their unity during last week's meetings to inaugurate new PSUV state governors - most of which began with the audience standing for a recording of Chavez singing the national anthem.
Cabello quipped that wealthy opposition leaders had been taking time out from Christmas holidays abroad to pray that he and Maduro would start fighting each other.
"They keep thinking I'll ignore the comandante's order," he scoffed. "He was very clear ... He chose comrade Nicolas Maduro, knowing that under him the revolution will not be lost."
Cabello and other senior PSUV officials, including Maduro's wife, Attorney-General Cilia Flores, have suggested Chavez's January 10 inauguration for his new six-year term could be postponed.
That would outrage the opposition and drive emotions on all sides even higher, but that date could prove to be a diversion.
Without any clarity on Chavez's condition, and the growing possibility that after four operations in 18 months he may no longer be fit to govern, many Venezuelans are expecting change and few are confident enough to make predictions.
"There are simply too many moving pieces," wrote Charles Shapiro, a former U.S. ambassador to Venezuela who now heads the San Diego-based Institute of the Americas think tank. "My bet is that a deeply polarized and de-institutionalized Venezuela will be both turbulent and unstable for the foreseeable future."
(Editing by Kieran Murray and Mohammad Zargham)