On a Sunday in December when the Ukrainian uprising seemed about to tip into wide-scale violence, Petro Poroshenko, a pro-Western billionaire, thrust himself between antigovernment protesters and riot police officers clashing outside the presidential headquarters, climbed on a bulldozer that was threatening to plow through the crowd and grabbed an orange plastic megaphone. “Friends, calm down,” he shouted, as pro-government thugs brought in to antagonize the demonstrators cursed him and hurled anti-Semitic slurs, though he is a member of the Ukrainian Orthodox faith, not Jewish. As chaos swirled, Mr. Poroshenko, 48, stood his ground, helping keep injuries to a minimum, but also cementing his status as a leader of the pro-European opposition and defying the stereotype of the superrich above it all. Now, with less than a week to go until a presidential election here, Mr. Poroshenko is once again at the center of a fracas […]