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Howard Dean to campaign for Hillary Clinton

Dean and Sanders have never been particularly close, despite having much in common, but his support for Clinton underscores Sanders' biggest problem.

Democratic presidential candidate Hillary Clinton waits to be introduced at a campaign event at the University of Wisconsin-Milwaukee on Sept. 10, 2015 in Milwaukee, Wis. (Photo by Scott Olson/Getty)
Democratic presidential candidate Hillary Clinton waits to be introduced at a campaign event at the University of Wisconsin-Milwaukee on Sept. 10, 2015 in Milwaukee, Wis.Getty Images

Alex Seitz-Wald