The tightening Democratic race reflects a modest but consistent decline in Obama's personal image rather than improved impressions of Clinton. Fewer Democrats ascribe positive qualities to Obama than did so a month ago, with white working-class Democrats, in particular, expressing more skeptical views of the Illinois senator. Since late February, his unfavorable rating has risen six points among all Democratic and Democratic-leaning voters. At the same time, Clinton's unfavorable rating among Democratic voters also has increased by seven points.

Compared with a month ago, race and class are now bigger drivers of preferences in the Democratic contest. Obama has lost ground among whites - especially white working-class voters - who now prefer Clinton by an even larger margin than they did in late March. Her lead among whites who did not attend college has increased from 10 points in March to 40 points today, and her lead among white Democrats who earn less than $50,000 a year has increased from two points to 24 points. Clinton has taken a 10-point lead among white male Democratic voters - erasing Obama's advantage with the group - and she now runs better among Democrats under age 50 than she had previously.