Hunter-Reay wrested the advantage away from Ryan Briscoe following the eighth of nine cautions on the 2.04-mile, 13-turn temporary street course in downtown Baltimore, then hung on for a 1.4-second win ahead of Briscoe.
It was a series-leading fourth win of the season for Hunter-Reay.
He came into Sunday's race 36 points behind series leader Will Power. But his victory, coupled with Power's sixth-place finish, cut his deficit to 17 points heading into the Sept. 15 IndyCar finale at Sonoma, Calif.
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Michael Sam cut from the Dallas Cowboys