In mid-January, a few weeks before the Iowa caucuses, Senator Bernie Sanders’s pollster offered a stark prognosis for the campaign: Mr. Sanders was on track to finish strong in the first three nominating states, but Joseph R. Biden Jr.’s powerful support from older African-Americans could make him a resilient foe in South Carolina and beyond.
Though Mr. Sanders climbed to a position of seeming dominance by mid-February, he and his inner circle also made a series of fateful decisions that left him ill positioned to win over skeptical Democrats — and sorely vulnerable to an opponent with Mr. Biden’s strengths.
Mr. Sanders proved unable to expand his base well beyond the left or to win over African-Americans in meaningful numbers.
He failed to heed warnings from traditional party leaders, and even from within his campaign, about the need to modulate his message and unify Democrats.
He allowed internal arguments to fester within his campaign, an ungainly operation that fragmented into factions beneath the only two real decision makers — Mr. Sanders and his wife, Jane.