(Adds background on constitution, final paragraph)

By Joseph Guyler Delva

PORT-AU-PRINCE, May 12 (Reuters) - Haitian lawmakers rejected President Rene Preval's candidate for prime minister on Monday, undercutting his efforts to establish a stable democracy in the deeply impoverished Caribbean country.

Ericq Pierre's nomination failed by a vote of 51-35, with nine members abstaining in the vote in the lower house of parliament, the Chamber of Deputies.

He had won overwhelming approval in Haiti's Senate last Wednesday and had been widely expected to succeed Jacques Edouard Alexis, who was fired by the Senate on April 12 after a week of food riots that killed at least six people. Senators said Alexis had not done enough to increase national food production and lower the cost of living in the poorest country in the Americas.

The rejection, on grounds that Pierre had failed to provide proof required under the constitution that he was descended from native-born Haitians, will force Preval to select another nominee in a process that could drag out for days or weeks.

Slum leaders in Les Cayes, the southern city where last month's food riots began, have threatened more violent protests if parliament did not install a new government by early this week.

There was speculation Pierre's rejection was organized by Alexis supporters seeking revenge for his dismissal. It also seemed aimed at Preval himself, who took office two years ago this week and has been widely criticized for moving too slowly to address the overwhelming needs of a country where most people live on less than $2 per day.

"He's been a disappointment to a lot of people," said Colette Lespinasse, the head of a U.N.-backed Haitian human rights organization.

"The government has had its head in the clouds while the country is stuck on the ground," she added, saying Preval had done little to fight poverty and make adequate use of international aid since taking office in May 2006.

TROUBLE AHEAD?

At least 20 members of Preval's own political party, Lespwa, voted against Pierre, who is a former adviser to the Inter-American Development Bank.

"Those who voted against Ericq Pierre should be held responsible for any trouble or unrest that may take place in the country in the coming days," said Saurel Francois, a deputy loyal to ousted former President Jean-Bertrand Aristide.

Haitians say the cost of some staples such as rice, beans and flour has doubled in the past few months.

Haiti has seen little but political upheaval and brutal dictatorship since it threw off French rule in a slave revolt more than 200 years ago. It stands among a number of poor countries rattled by violence over escalating food prices blamed on growing demand in Asia, diversion of crops for biofuel, record oil prices and market speculation.

Preval, who took office in May 2006, also served as president from 1996 to 2001 and is the only elected Haitian leader to serve a full term and successfully hand over power to a democratically elected successor.

In his first term, it took Preval 21 months to put a new government in place after then-Prime Minister Rosny Smarth resigned in June 1997.

Pierre was rejected by lawmakers as a candidate for prime minister during Preval's first term as well. Both times it was because he lacked birth certificates for his grandparents proving they were natives of Haiti.

That requirement for holding high office, an obstacle for many potential leaders in a country with a shoddy history of public record-keeping, was written into the constitution in the late 1980s when Haiti was ruled by a military dictatorship. (Additional reporting by Tom Brown; Editing by Peter Cooney)

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