By Angeliki Koutantou and Harry Papachristou ATHENS (Reuters) - The main party in Greece's ruling coalition, conservative New Democracy, has nudged ahead of the leftist opposition for the first time in six months, an opinion poll showed on Saturday, ahead of a key parliamentary vote on more tough reforms. The poll will be welcome news for Prime Minister Antonis Samaras' fragile government, whose cohesion has been tested by the controversial reforms demanded by international lenders as a condition to keep debt-laden Greece afloat. The poll, conducted on March 24-27 by Alco for weekly newspaper Proto Thema, showed Samaras' New Democracy winning 21.7 percent of the vote if elections were held today, against 21.2 percent for the main opposition, anti-bailout Syriza party. In Alco's previous poll on March 11-13, Syriza led New Democracy by 21.2 to 20 percent, a lead similar to that in all other polls tracked by Reuters in recent months.