WASHINGTON — Senator Barack Obama’s support among Democrats nationally has softened over the last month, particularly among men and upper-income voters, as voters have taken a slightly less positive view of him than they did after his burst of victories in February, according to the latest New York Times/CBS News poll. The survey suggests that Mr. Obama, Democrat of Illinois, may have been at something of a peak in February, propelled by a string of primary and caucus victories over Senator Hillary Rodham Clinton of New York, and that perceptions of him are settling down. Mr. Obama’s favorability rating among...