Sen. Bernie Sanders’ surprise victory over former Secretary of State Hillary Clinton in Tuesday’s Michigan Democratic presidential primary came after many pollsters predicted she would win handily.

The average of 18 major polls prior to Tuesday’s primary indicated Clinton would win by 21.4%, according to an analysis by RealClear Politics.

“We all blew it,” said Steve Mitchell, CEO of Mitchell Research & Communications of East Lansing, Mich., in an interview Wednesday. Mitchell’s firm showed a separation of 28 percentage points several days before the primary, based on an automated land line phone survey of 427 likely Democratic voters.

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