Pole-sitter Power ended the day only five points in back of Franchitti after relinquishing the lead only briefly during the 75-lap race on the 2.4-mile, 12-turn temporary street circuit in downtown Baltimore.
Power claimed his sixth win of the season and the 15th of his IndyCar career.
Franchitti finished fourth with just three remaining races left in the series to nail down his second straight points title.
Finishing second was Oriol Servia and Tony Kanaan -- who was involved in crash with Helio Castroneves during warm-ups Sunday -- took third.
Franchitti's Ganassi Racing teammate Scott Dixon finished fifth.