By Angeliki Koutantou ATHENS (Reuters) - Greece's Syriza party is on course to win 26 percent of votes in a snap election in September, just one point ahead of the conservative New Democracy party, a poll published by the Pulse for Action 24 television channel said on Tuesday. Former premier Alexis Tsipras of Syriza resigned in August to seek a new mandate for a bailout deal he clinched with the country's international creditors, but Syriza's slipping poll lead suggests the decision could backfire. Tsipras came to power in January promising to end austerity in a country battered by recession and unemployment but eventually accepted stringent conditions for a new 86-billion-euro bailout after months of difficult negotiations with Greece's lenders.