* Leftist promises to honor contracts, rule of law
* Humala has fallen behind Fujimori in polls
* Prominent intellectuals rally behind Humala
* Vargas Llosa says choice between democracy, dictatorship (Updates with Humala speech, endorsement of intellectuals)
By Terry Wade and Patricia Velez
LIMA, May 19 (Reuters) - Left-wing nationalist Ollanta Humala publicly pledged on Thursday to respect democracy and the market economy if elected president, angling for support from centrists as a poll showed him trailing before the June 5 vote.
It was the latest attempt by Humala to assuage fears that he has not fully abandoned his radical past -- or that he might try to change the constitution to stay in power if elected and roll back years of free-market reforms in Peru's booming economy.
A Datum survey showed his right-wing opponent Keiko Fujimori leading with 52.1 percent of voter support against 47.9 percent for Humala, the sixth recent poll to show Fujimori ahead.
"I swear before God and my people that I will fulfill these promises and only act within the current constitutional order and the rule of law," he said in a speech attended by prominent intellectuals in which he vowed to respect the constitution, private enterprise and a free press.
Nobel prize-winning writer Mario Vargas Llosa, who addressed the group by a video feed, urged Peruvians to back Humala.
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Fujimori's critics say she would govern like her father, jailed former president Alberto Fujimori, whose authoritarian government collapsed in 2000 in a cloud of corruption scandals and charges of human rights abuses.
"I think Humala's statement and his revised government plan should vanquish any doubts that still win undecided voters," Vargas Llosa said. "I implore you to vote for Humala to defend democracy in Peru and avoid the scenario of a new dictatorship."
Humala revised his government plan last week to make it more attractive to investors, dropping a controversial tax increase and a proposal to take over private pension funds.
To woo centrists, Humala has tried to distance himself from his former political mentor, fiery Venezuelan President Hugo Chavez, and recast himself as a moderate like Brazil's popular former President Luiz Inacio Lula da Silva.
But Fujimori has overtaken him in the recent weeks in the opinion polls, with Thursday's survey showing him struggling to make inroads among undecided voters. The mock vote organized by Datum surveyed 1,211 people nationwide on May 15 and has a margin of error of 2.8 percentage points.
Peru's benchmark stock index <.IGRA> and sol currency <PEN=PE> were little changed on the news as many traders have already bet on a victory by Fujimori.
FATHER IN PRISON
Both candidates appeal to poorer voters, though Fujimori has support from the business community, which is eager to see a decade-long boom continue and worries Humala might adopt policies that hurt private investment.
Fujimori's father opened up the economy to foreign investment and brought hyperinflation under control, but his government collapsed in 2000 in a cloud of corruption and human rights scandals stemming from his crackdown on insurgents.
Keiko Fujimori has apologized for the excesses of her father's government, promised to respect human rights and says she would not seek his release from jail.
The Datum poll showed 64.5 percent of voters believe she would free him anyway.
Humala, who led a short-lived revolt against Fujimori's father while in the army, campaigns as a moderate leftist but spooks investors with his more nationalist policy platform that outlines a more interventionist agenda.
According to Datum, 60.3 percent of voters think his tamed campaign rhetoric is merely an election strategy.
Humala says his brand of nationalism would strengthen a weak state to make sure the benefits of economic growth reach all Peruvians, not just local elites or foreign firms. A third of Peruvians still live in poverty.
Despite his pledges to be conciliatory, some members of his Nationalist Party shun international cooperation and advocate a brand of ethnic nationalism that harks back to Peru's Inca past and seeks to turn the country's back on the global economy. (Reporting by Patricia Velez, Terry Wade and Marco Aquino; editing by Anthony Boadle)
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