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Hillary Rodham Clinton
Though she has said nothing definite about the possibility of a presidential run, speculation points to her entering the political ring one more time. (Barring an unexpected shift, she'll be leaving her position as secretary of State.) With her husband, former President Clinton, playing a prominent (and perhaps decisive) role in President Obama's reelection, the Clintons are back in the national consciousness, which is only fueling the assumption that the 2008 Democratic primary runner-up will be at the front of the pack in 2016. But a recent series of health concerns, ranging from a concussion to a blood clot, have raised new questions about Clinton's viability in 2016.