* Election in April/May, polls favour leftBy Brian LovePARIS, Oct 16 (Reuters) - French left-wingers began voting on Sunday to decide who will lead their challenge to unseat President Nicolas Sarkozy in an election next year, with moderate Socialist Party veteran, Francois Hollande the favourite to win.In a U.S.-style primary, the first of its kind in France, voters will choose between Hollande, who has never held a national government post, and Martine Aubry, one-time labour minister, architect of France’s 35-hour week and daughter of the former European Commission President Jacques Delors.Opinion polls give Hollande a lead of six percentage points over Aubry in a ballot that decides which of the two will run in a presidential contest that the Socialists have not won since Francois Mitterand was re-elected in 1988.The polls suggest French voters are ready to put the left back in power after five years of conservative Sarkozy, who is unpopular but widely expected to seek another five-year term.The left’s runaway favourite to become president had been former International Monetary Fund chief Dominique Strauss-Kahn but his IMF career and presidential hopes were halted when he was arrested in New York in May on charges of sexually assaulting a hotel maid. The charges have since been dropped.The ease with which Hollande and Aubry have filled his shoes suggests that many voters are simply weary of Sarkozy and his economic policies.Voters living abroad or in some French overseas territories were first to cast their ballots and the bulk of almost 10,000 polling stations opened at 0700 GMT on mainland France. They were set to close at 1700 GMT , with p reliminary results are expected a few hours later.CONCILIATORY NOTEHollande and Aubry sparred in the days before the primary but Aubry seized on France’s World Cup rugby semi-final win over Wales to sound a conciliatory note ahead of Sunday’s vote.”When it’s time for the post-match session, everyone parties together,” she told reporters. “That’s how it’ll be on Monday.”She dismissed polls that show Hollande scoring 53 percent of the vote to her 47 percent, preferring to highlight declarations of support from several prominent environmentalist politicians.In a primary inspired by the momentum that carried Barack Obama to the White House, the Socialist Party has organised a two-round contest where anyone who pays a euro and declares allegiance to left-wing values can vote.More than 2.6 million people voted in the first-round last Sunday, when anti-globalisation hardliner Arnaud Montebourg scored a surprise 17 percent.Hollande, who promised in the ensuing days to crack down on banks and financial market excess, has consolidated his position versus Aubry by securing the support of the four contenders knocked out in round one, including Montebourg.Hollande, seen by many as more centre-left, won 39 percent of the first-round vote, versus 30 percent for Aubry, often labelled as a more old-school Socialist. The four candidates knocked out — including Segolene Royal, Hollande’s former companion and mother of his four children — got close to 30 percent.But both Hollande and Aubry share the main tenets of a Socialist Party manifesto that promises to scrap 50 billion euros of tax breaks that mostly went to the wealthy under Sarkozy, using half of this money to fund state jobs and promote growth, with the rest to cut the deficit.Sarkozy, who took power in 2007 after 12 years of Jacques Chirac, has yet to declare a re-election bid.Opinion polls show him trailing either Hollande or Aubry in the election which takes place in two rounds on April 22 and May 6, followed weeks later by a parliamentary election.