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DETROIT, MI - MAY 4:  Republican Dr. Ben Carson, a retired pediatric neurosurgeon, speaks as he officially announces his candidacy for President of the United States at the Music Hall Center for the Performing Arts May 4, 2015 in Detroit, Michigan. Carson was scheduled to travel today to Iowa, but changed his plans when his mother became critically ill. He now will be traveling to Dallas instead to be with his mother Sonya. (Photo by Bill Pugliano/Getty Images)
DETROIT, MI - MAY 4:  Republican Dr. Ben Carson, a retired pediatric neurosurgeon, speaks as he officially announces his candidacy for President of the United States at the Music Hall Center for the Performing Arts May 4, 2015 in Detroit, Michigan. Carson was scheduled to travel today to Iowa, but changed his plans when his mother became critically ill. He now will be traveling to Dallas instead to be with his mother Sonya. (Photo by Bill Pugliano/Getty Images)

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    Rival surges, ties Donald Trump in Iowa poll

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Rival surges, ties Donald Trump in Iowa poll

Dr. Ben Carson is tied with Donald Trump in the battleground state of Iowa in a Monmouth University poll. CNN's Don Lemon speaks to Robert Taylor from Carson's campaign.

Rival surges, ties Donald Trump in Iowa poll

Dr. Ben Carson is tied with Donald Trump in the battleground state of Iowa in a Monmouth University poll. CNN's Don Lemon speaks to Robert Taylor from Carson's campaign.