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Voters began heading to the polls early on Tuesday morning in Struthers, Ohio, a struggling town near the eastern city of Youngstown, where the economy is a key campaign issue.
Ohio is the largest battleground and most closely watched prize of the 10 contests on Super Tuesday.
Other state contests include Virginia, Massachusetts, Tennessee, Georgia, Alaska, Idaho, North Dakota, Oklahoma, and Vermont.
Super Tuesday is the biggest day so far in the Republican race to find a challenger to President Barack Obama in the November 6 election. At stake are 419 of the 1,144 delegates needed to win the party's nomination.