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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. It helps that many Democrats have already pledged their support for Clinton's yet-unannounced campaign, and she sparked a social media wildfire Monday with the debut of her Twitter account. 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.