Twitter and Facebook are scrambling to assuage conservative leaders who have sounded alarms - and sought to rile voters - with accusations that the country's tech giants are censoring right-leaning posts, tweets and news.From secret dinners with conservative media elite to private meetings with the Republican National Committee, the new outreach reflects tech giants' delicate task: satisfying a party in power while defending online platforms against attacks that threaten to undermine the public's trust in the web.