AUGUSTA, Maine — Maine’s ranked-choice voting system could boost Democrats in targeted races for U.S. Senate and one presidential elector from the swing 2nd Congressional District as they hold narrow leads over Republicans, according to a poll released Thursday.The poll, conducted by SurveyUSA for the electoral reform group FairVote and shared exclusively with the Bangor Daily News, shows House Speaker Sara Gideon with a one-percentage-point lead over fourth-term Sen.