* Key Socialists defect to centrist candidate
* Divisions call into question Socialist Party's survival
By Brian Love
PARIS, March 30 (Reuters) - More than one in two French voters believe struggling Socialist candidate Benoit Hamon should drop out of the presidential race in favour of a rival left-winger who has overtaken him in surveys of voting intentions, a poll showed on Thursday.
The Harris Interactive poll was conducted after Wednesday's announcement by former prime minister Manuel Valls that he would join a growing number of fellow Socialists who refuse to vote for Hamon, their own party's official candidate.
The disarray on the left has called into question the survival of the Socialist Party and cleared a path for independent centrist Emmanuel Macron and far-right leader Marine Le Pen to emerge as favourites to win the presidency.
Hamon's problems partly reflect the depth of public disenchantment with the Socialists after five years of rule by Socialist President Francois Hollande.
Hamon, livid at Valls' decision to join other key Socialists defecting to Macron, brushed off talk of an early election race exit, saying in an interview on public service radio channel franceinfo: "Of course I will stay through to the finish."
Two left-wingers, one from the Socialist Party and one not, are fourth and fifth in the polls less than a month before the first round of the election on April 23. Only the two top candidates go through to the May 7 runoff.
Skirmishing in the Socialist Party, the largest party on the Left, risks widening age-old divisions between moderates like Valls and hardline left-wingers like Hamon to breaking point under election pressures.
The Harris Interactive poll found that 53 percent of voters felt Hamon should pull out of the race in favour of Jean-Luc Melenchon, firebrand candidate of the Left Party, or what the French call "the Left of the Left"
Melenchon has overtaken Hamon in a number of polls in the last two weeks but the split of the left-wing vote means neither is likely to make it past the first round.
The favourite, Macron, is tipped in all polls to easily beat Le Pen in the May 7 runoff.
Erstwhile frontrunner Francois Fillon is in third place and faces elimination too.
Hamon has for weeks been urging Melenchon to unite with the Socialists behind him as a single candidate. But Melenchon again ruled that out overnight, telling supporters at a rally in the northern port of Le Havre: "I will negotiate with nobody."
If there were one rather than two left-wing candidates, that contender would have a chance on paper of qualifying for the May 7 runoff if he managed to combine some 25 percent of votes now split between the two.
Reporting by Brian Love; Editing by Adrian Croft