By Brian Winter and Anthony Boadle
BRASILIA, May 1 (Reuters) - It's the catch phrase of the moment in Brazil's capital, seen on posters and even a few bumper stickers: "Come back, Lula."
As President Dilma Rousseff sags in polls ahead of this October's election, there are growing calls for her popular predecessor and mentor Luiz Inacio Lula da Silva to take her place as the Workers' Party candidate.
Lula, who presided over an economic boom as president from 2003 to 2010, remains Brazil's most popular politician by far. His legendary schmoozing ability and pragmatic policies are a source of nostalgia among many investors and others frustrated with Rousseff's more hermetic personal style and heavy hand in the economy, which has sputtered on her watch.
The clamor for the 68-year-old former metalworker's return has gained support in Congress, including among members of Rousseff's own coalition.
Twenty legislators from the mid-sized Republic Party signed a manifesto this week saying that Lula's leadership was necessary "at this time of crisis in Brazil and abroad."
Party whip Bernardo Vasconcellos hung a photo of Lula in the party's office and told reporters: "It's not that we don't want (Rousseff); We want Lula."
Sources close to both leaders told Reuters that, despite the groundswell, Rousseff is almost certain to be the Workers' Party candidate this year, although they declined to totally rule out Lula's return.
Speaking on condition of anonymity, they attributed the "Come back, Lula" movement to natural pre-election jitters and jockeying by parties seeking favors ahead of the election.
Despite her recent slide in polls, Rousseff retains a lead of about 15 percentage points over her nearest rival in opinion polls and remains very popular among Brazil's poor.
Lula has repeatedly declared in public that he will not run, and will support Rousseff.
Nevertheless, when asked if there was any scenario under which Rousseff would step aside as a candidate in October, one source replied: "Only if there's a catastrophe."
That source declined to elaborate. But government officials have worried about several potential problems before the election, including a further downturn in the economy; an embarrassing logistical meltdown during the World Cup in June and July; or widespread, violent street protests during the soccer tournament, similar to a series of demonstrations that rocked the country last year.
A congressional inquiry into alleged mismanagement at Petroleo Brasileiro SA could also bring complications, since Rousseff was the state-run oil company's chairwoman before becoming president.
The Workers' Party is expected to formally launch Rousseff's candidacy at a party convention in June. Under Brazilian electoral law, however, the party could switch candidates until about three weeks before the Oct. 5 vote.
Jose Chrispiniano, Lula's spokesman, said via e-mail: "In the 2014 elections, Lula will not be a candidate. He will be an electoral sponsor of President Dilma Rousseff."
Given Brazil's recent history, a renewed clamor for Lula was probably inevitable.
Helped by high prices for its commodities and a boom in consumer credit, Brazil's economy grew a torrid 7.5 percent in 2010, Lula's final year in office. Overall, during his presidency, some 30 million people were pulled out of poverty.
Those triumphs allowed Lula to leave office with approval ratings upward of 80 percent. Prevented by law from running for a third consecutive term, he convinced voters to elect Rousseff, a lifelong civil servant who was his chief of staff but had never run for public office before.
Almost as soon as Rousseff took office, Brazil's economy slowed dramatically. Some economists blamed Lula, saying that by gunning the economy's engines with cheap loans from state banks, he left an unpleasant cleanup for his successor.
Others blame softer demand for commodities from China, and Rousseff's habit of intervening in the private sector more than Lula did. She has alternately raised and cut taxes and engineered drops in interest rates and electricity prices in a mostly fruitless effort to get the economy going again.
With each recent poll showing declines in Rousseff's support, financial markets have gone up as many investors hope that one of her rivals - or Lula - will succeed her.
The clamor grew so loud this week that even Rousseff was compelled to publicly address it.
"Nothing can separate me from him," she told reporters. "I know of his loyalty to me, and he knows of my loyalty to him."
"I'd like it if, when I'm a candidate, I had support from my base," she said in a separate radio interview Wednesday. "But, even without that support, we're still going to go forward."
Most political analysts still believe that Rousseff will be the candidate, and see her as the clear favorite to win. They say that switching candidates so late would taint the sense of inevitable victory that still prevails among many voters.
Lula's own viability as a candidate is uncertain, in part because of health concerns. He has recovered from throat cancer diagnosed in 2011, doctors say, but he spent the night in a hospital just last week because of an inner ear infection.
Rousseff, too, is a cancer survivor, and some analysts have speculated that any turn for the worse in her own health could be another scenario that would warrant Lula's return.
Still, Lula realizes that a third term could damage his legacy, sources close to him have said. They add that, if he were to run again, he's more likely to do so in 2018.
Andre Cesar, a political analyst, said that if Rousseff's support drops below 30 percent in polls in coming weeks, the party would likely consider Lula as an option.
A poll released this week showed Rousseff with 37 percent support, down from 44 percent in February. Her more centrist opponents, Senator Aecio Neves and former governor Eduardo Campos, had 22 percent and 12 percent support, respectively.
The rest of the vote was split among undecided voters and those who said they would spoil their ballot in protest.
(Editing by Todd Benson and Kieran Murray)