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In particular, Mr. Obama has been focusing on young voters, a key voting bloc in his presidential campaign but one that’s historically less likely to show up to vote in non-presidential races. To help counter that trend, Mr. Obama is hosting another rally Sunday, this time in Philadelphia and featuring hip-hop group the Roots, which hit the trail with him in 2008.

The One Nation event, put together by progressive groups, unions and civil rights organizations, followed a rally hosted by Mr. Obama at the University of Wisconsin at Madison that drew 26,000 people. Vice President Joseph R. Biden Jr. also drew an overflow crowd at Penn State University last week.