(Reuters) - With Russia's incursion into Ukraine reviving Cold War-style tensions, President Barack Obama is at risk of suffering a blow to his credibility at a time when he can least afford it: as he tries to convince voters to stick with his fellow Democrats in congressional elections that will help shape his legacy. For five years, Obama has practiced a cautious approach to foreign policy crises, prizing sober diplomacy and the search for consensus over brinkmanship, in prolonged conflicts in Iraq and Afghanistan. But the deliberative style that Obama's team sees as a statesmanlike attitude in tune with Americans'...