The former congressman met with supporters outside Casa Sunzal, a Southwest Key facility near downtown..
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He said the children were fleeing violence.
He then compared their situation to the United States turning away Jewish refugees and Japanese Americans during World War II.
Southwest Key employees say Casa Sunzal is a shelter, not a detention facility, that seeks to unify immigrant children with their families.
O'Rourke will join other Democratic candidates in Houston Friday for a Presidential forum hosted by the National Education Association.
The group says former Vice President Joe Biden, Julian Castro, Governor Jay Inslee and Senators Amy Klobuchar, Bernie Sanders and Elizabeth Warren have all confirmed their attendance.
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They will be asked questions from educators about the future of public education.